Add README.md

The repository is now public [1], thus add a `README.md` file.

Link: https://lore.kernel.org/rust-for-linux/CANiq72kMS8UoYpg0hW1=UDAAkytEoeyqGzg+Sys8VSaucr+HzQ@mail.gmail.com/ [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

README.md

@@ -0,0 +1,5 @@
+# rust-for-linux.com
+
+These are the sources for the [rust-for-linux.com](https://rust-for-linux.com) website.
+
+This repository is primarily meant for Rust for Linux team members.