Add dummy file to fix symlink (apache#3759)

In `assets/static` we have a symlink pointing to `docs/_build/html`
which is .gitignored and missing in the repo since we don't want the
html-generated docs in the repo, meaning the symlink is broken

Having the symlink broken creates issues with pip-compile somehow.

Adding a dummy file here addresses that

docs/_build/html/README.md

@@ -0,0 +1 @@
+Folder containing the sphinx-generated documentation