Start writing some docs, in particular, an "intro to Mojo".

docs/README.md

Index: docs/README.md
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+# Documents
+
+This directory (tree) contains documentation about Mojo.
+
+* [Introduction to Mojo](intro/intro.md)
+
+## Notes
+
+Documentation under this directory should be in Markdown whenever possible. It
+should generally adhere to the [Google documentation
+guide](https://github.com/google/styleguide/tree/gh-pages/docguide), including
+in particular the [Markdown style
+guide](https://github.com/google/styleguide/blob/gh-pages/docguide/style.md).