install-compile-source.md: also explain building snapshots
install-compile-source.md: also explain building snapshots

The question on how to build a snapshot version of bazel arises
frequently, when a fix for a bug is submitted and people want to
try the fixe immediately. Therefore, add a paragraph to the
compile-from-source instructions, explaining how to do this.

Change-Id: I03b26989c3288e270a6efecf73634ed46cdcfb50
PiperOrigin-RevId: 211056412

dslomov/bazel,meteorcloudy/bazel,ButterflyNetwork/bazel,akira-baruah/bazel,twitter-forks/bazel,meteorcloudy/bazel,davidzchen/bazel,perezd/bazel,bazelbuild/bazel,dslomov/bazel,katre/bazel,bazelbuild/bazel,bazelbuild/bazel,twitter-forks/bazel,dslomov/bazel-windows,ulfjack/bazel,meteorcloudy/bazel,davidzchen/bazel,cushon/bazel,safarmer/bazel,aehlig/bazel,ButterflyNetwork/bazel,bazelbuild/bazel,perezd/bazel,perezd/bazel,aehlig/bazel,werkt/bazel,davidzchen/bazel,ulfjack/bazel,cushon/bazel,perezd/bazel,akira-baruah/bazel,dslomov/bazel-windows,twitter-forks/bazel,werkt/bazel,perezd/bazel,cushon/bazel,safarmer/bazel,dslomov/bazel,dslomov/bazel-windows,dslomov/bazel-windows,twitter-forks/bazel,bazelbuild/bazel,ulfjack/bazel,davidzchen/bazel,ButterflyNetwork/bazel,davidzchen/bazel,perezd/bazel,akira-baruah/bazel,ulfjack/bazel,safarmer/bazel,dslomov/bazel-windows,dslomov/bazel,davidzchen/bazel,cushon/bazel,perezd/bazel,davidzchen/bazel,meteorcloudy/bazel,werkt/bazel,dslomov/bazel,dslomov/bazel,meteorcloudy/bazel,aehlig/bazel,akira-baruah/bazel,ulfjack/bazel,twitter-forks/bazel,aehlig/bazel,bazelbuild/bazel,twitter-forks/bazel,cushon/bazel,cushon/bazel,akira-baruah/bazel,dslomov/bazel-windows,ulfjack/bazel,twitter-forks/bazel,akira-baruah/bazel,katre/bazel,ButterflyNetwork/bazel,safarmer/bazel,dslomov/bazel,ButterflyNetwork/bazel,aehlig/bazel,ulfjack/bazel,aehlig/bazel,katre/bazel,werkt/bazel,werkt/bazel,meteorcloudy/bazel,ButterflyNetwork/bazel,katre/bazel,katre/bazel,aehlig/bazel,safarmer/bazel,safarmer/bazel,meteorcloudy/bazel,katre/bazel,werkt/bazel