[docs] Guide prospective TableGen backend writers.

Boilerplate is often the hardest part of getting started with these
kinds of things, so throw them a bone.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173969 91177308-0d34-0410-b5e6-96231b3b80d8
Remove a mention of TargetInstrDescriptor, which no longer exists in the code


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173971 91177308-0d34-0410-b5e6-96231b3b80d8