[docs] Adding "new target" rules to dev policy

Making explicit our current policy to accept new targets as experimental and
later official. Every new target should follow these rules to be added,
and kept relevant in the upstream tree.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278971 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed