[docs] Add a note to docs/README.txt

Added note to docs/README.txt on how to check the reachibility of
external links in the documentation.

Patch by Dan Liew!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206924 91177308-0d34-0410-b5e6-96231b3b80d8