Update README.txt to more accurately reflect reality.

This closes PR14709 and hopefully is a small step towards preventing
similar issues in the future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171837 91177308-0d34-0410-b5e6-96231b3b80d8

README.txt

@@ -8,8 +8,10 @@ optimizers, and runtime environments.
 LLVM is open source software. You may freely distribute it under the terms of
 the license agreement found in LICENSE.txt.
 
-Please see the HTML documentation provided in docs/index.html for further
-assistance with LLVM.
+Please see the documentation provided in docs/ for further
+assistance with LLVM, and in particular docs/GettingStarted.rst for getting
+started with LLVM and docs/README.txt for an overview of LLVM's
+documentation setup.
 
-If you're writing a package for LLVM, see docs/Packaging.html for our
+If you're writing a package for LLVM, see docs/Packaging.rst for our
 suggestions.