README: Add documentation build status badge. #2345

Job	Run time
linux rpm fedora	9m 4s
osx clang --disable-ssl	3m 11s
dpdk gcc	8s
linux deb no dpdk	8m 38s
linux gcc --disable-ssl	4m 32s
linux clang --disable-ssl	6m 35s
linux gcc c99	5m 9s
linux clang c99	3m 55s
linux gcc test	9m 57s
linux clang test asan	19m 15s
linux clang test ubsan	14m 17s
linux gcc test --enable-shared	19m 4s
linux clang test --enable-shared	15m 22s
linux gcc test dpdk	13m 24s
linux clang test dpdk	13m 0s
linux gcc test -ljemalloc	12m 27s
linux clang test -ljemalloc	12m 55s
linux gcc --enable-afxdp	4m 9s
linux clang --enable-afxdp	3m 14s
linux gcc dpdk --enable-shared	6m 23s
linux clang dpdk --enable-shared	4m 20s
linux gcc dpdk-shared	4m 33s
linux clang dpdk-shared	4m 20s
linux gcc dpdk-shared --enable-shared	6m 28s
linux clang dpdk-shared --enable-shared	3m 58s
linux gcc m32 --disable-ssl	3m 56s
	3h 32m 14s

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