Write tutorial text for identity element

See gst-plugin-tutorial/src/identity.rs for the code.

A tutorial-style text like https://coaxion.net/blog/2018/01/how-to-write-gstreamer-elements-in-rust-part-1-a-video-filter-for-converting-rgb-to-grayscale/ and https://coaxion.net/blog/2018/02/how-to-write-gstreamer-elements-in-rust-part-2-a-raw-audio-sine-wave-source/ should be written for these too.

Se also #44 (closed)