[mlir] Add short readme.txt to docs directory

[jpienaar]

Summary

Refer folks to the main website and make it explicit that the rendered
output is what is of interest and that the GitHub viewing experience may
not match (even though we are trying to keep it as close as possible, the
renderers do differ).

mlir/docs/README.txt

+MLIR documentation
+==================
+
+Please note mlir.llvm.org is where MLIR's rendered documentation is displayed.
+The viewing experience on GitHub or elsewhere may not match those of the
+website. For any changes please verify instead that they work on the main
+website first.
+
+See https://github.com/llvm/mlir-www for the website generation information.