Add release update instructions for libc++

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290444 91177308-0d34-0410-b5e6-96231b3b80d8

RELEASE.TXT

@@ -0,0 +1,11 @@
+//===---------------------------------------------------------------------===//
+// Post-Release TODO
+//===---------------------------------------------------------------------===//
+
+These notes contain a list of things that must be done after branching for
+an LLVM release.
+
+1. Update _LIBCPP_VERSION in `__config`
+2. Update the __libcpp_version file.
+3. Update the version number in `docs/conf.py`
+4. Create ABI lists for the previous release under `lib/abi`