add link to Custom Maps wiki

README.md

@@ -265,7 +265,7 @@ You will need to logout and login for the launcher to take effect.
 |![static map](https://github.com/semuconsulting/PyGPSClient/blob/master/images/staticmap.png?raw=true)| Map widget with various modes of display. Map Type = 'world': a static offline Mercator world map showing current global location.
 |![webmap widget](https://github.com/semuconsulting/PyGPSClient/blob/master/images/webmap_widget.png?raw=true)| Map Type = 'map' or 'sat': Dynamic, online web map or satellite image via MapQuest API (*requires an Internet connection and free [Mapquest API Key](#mapquestapi)*). Left Click +/- to zoom in or out. Right click +/- to zoom in or out to maximum extent. By default, the web map will automatically refresh every 60 seconds (*indicated by a small timer icon at the top left*). The default refresh rate can be amended by changing the `"mapupdateinterval_n":` value in your json configuration file, but **NB** the facility is not intended to be used for real-time navigation. Double-click anywhere in the map to immediately refresh. |
 |![custom map](https://github.com/semuconsulting/PyGPSClient/blob/master/images/custommap.png?raw=true)| Map Type = 'custom': One or more user-defined custom offline maps can be imported using the Menu..Options..Import Custom Map facility, or by manually setting the `usermaps_l` field in the json configuration file. The `usermaps_l` setting represents a list of map paths and bounding boxes in the format ["path to map image", [minlat, minlon, maxlat, maxlon]] - see [example configuration file](https://github.com/semuconsulting/PyGPSClient/blob/master/pygpsclient.json). Map images must be a [supported format](https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html) and use a standard WGS84 Web Mercator projection e.g. EPSG:4326.|
-|![import custom map](https://github.com/semuconsulting/PyGPSClient/blob/master/images/importcustommap.png?raw=true)| <a name="custommap">Import Custom Map dialog</a>. Click ![load icon](https://github.com/semuconsulting/PyGPSClient/blob/master/src/pygpsclient/resources/iconmonstr-folder-18-24.png?raw=true) to open the custom map image location (*the default file suffix is `*.gif` - select Show Options to select any file suffix `*.*`*). If the `rasterio` library is installed and the image is georeferenced (e.g. using [QGIS](https://qgis.org/)), the map extent will be automatically extracted - otherwise it must be entered manually. Import the custom map path anad extent settings by clicking ![play icon](https://github.com/semuconsulting/PyGPSClient/blob/master/src/pygpsclient/resources/iconmonstr-arrow-12-24.png?raw=true). See [CUSTOM_MAPS](https://github.com/semuconsulting/PyGPSClient/blob/master/CUSTOM_MAPS.md) for tips on how to create a suitable georeferenced map image.|
+|![import custom map](https://github.com/semuconsulting/PyGPSClient/blob/master/images/importcustommap.png?raw=true)| <a name="custommap">Import Custom Map dialog</a>. Click ![load icon](https://github.com/semuconsulting/PyGPSClient/blob/master/src/pygpsclient/resources/iconmonstr-folder-18-24.png?raw=true) to open the custom map image location (*the default file suffix is `*.tif` - select Show Options to select any file suffix `*.*`*). If the `rasterio` library is installed and the image is georeferenced (e.g. using [QGIS](https://qgis.org/)), the map extent will be automatically extracted - otherwise it must be entered manually. Import the custom map path anad extent settings by clicking ![play icon](https://github.com/semuconsulting/PyGPSClient/blob/master/src/pygpsclient/resources/iconmonstr-arrow-12-24.png?raw=true). See [Creating Custom Maps for PyGPSClient](https://www.semuconsulting.com/gnsswiki/custommapwiki/) for tips on how to create a suitable georeferenced map image.|
 |![spectrum widget](https://github.com/semuconsulting/PyGPSClient/blob/master/images/spectrum_widget.png?raw=true)| Spectrum widget showing a spectrum analysis chart (*GNSS receiver must be capable of outputting UBX MON-SPAN messages*). Clicking anywhere in the spectrum chart will display the frequency and decibel reading at that point. Double-clicking anywhere in the chart will toggle the GNSS frequency band markers (L1, G2, etc.) on or off. Right-click anywhere in the chart to capture a snapshot of the spectrum data, which will then be superimposed on the live data. Double-right-click to clear snapshot. **NB:** Some receivers (e.g. NEO-F10N) will not output the requisite MON-SPAN messages unless the port baud rate is at least 57,600. |
 |![sysmon widget](https://github.com/semuconsulting/PyGPSClient/blob/master/images/sysmon_widget.png?raw=true)| System Monitor widget showing device cpu, memory and I/O utilisation (*GNSS receiver must be capable of outputting UBX MON-SYS and/or MON-COMMS messages*). Tick checkbox to toggle between actual (cumulative) I/O stats and pending I/O. |
 |![scatterplot widget](https://github.com/semuconsulting/PyGPSClient/blob/master/images/scatterplot_widget.png?raw=true)| Scatterplot widget showing variability in position reporting over time. (Optional) Enter fixed reference position. Select Average to center plot on dynamic average position (*displayed at top left*), or Fixed to center on fixed reference position (*if entered*). Check Autorange to set plot range automatically. Set the update interval (e.g. 4 = every 4th navigation solution). Use the range slider or mouse wheel to adjust plot range. Right-click to set fixed reference point to the current mouse cursor position. Double-click clear existing plot. The default center mode, autorange, update increment, scale index and fixed reference position settings can be stored in the json configuration file as `scattercenter_s`/`scatterautorange_b`/`scatterinterval_n`/`scatterscale_n`/`scatterlat_f`/`scatterlon_f`. |