README: Add build badges. #2390

Job	Run time
osx clang --disable-ssl	3m 31s
linux rpm fedora	6m 32s
dpdk gcc	6s
linux gcc --disable-ssl	1m 52s
linux clang --disable-ssl	2m 3s
linux gcc test -300	8m 44s
linux gcc test 301-600	5m 58s
linux gcc test 601-	5m 30s
linux clang test sanitizers -300	11m 44s
linux clang test sanitizers 301-600	8m 58s
linux clang test sanitizers 601-	10m 54s
linux gcc test -ljemalloc -300	9m 4s
linux gcc test -ljemalloc 301-600	6m 7s
linux gcc test -ljemalloc 601-	4m 6s
linux gcc system-test-dpdk dpdk -100	12m 52s
linux gcc system-test-dpdk dpdk 101-	12m 3s
linux gcc system-test-userspace -100	11m 29s
linux gcc system-test-userspace 101-	10m 52s
linux gcc system-test -100	14m 35s
linux gcc system-test 101-	12m 33s
linux clang system-test sanitizers -100	16m 12s
linux clang system-test sanitizers 101-	14m 15s
linux x86 gcc --disable-ssl	1m 54s
	3h 11m 54s

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