add asciinema README to explain where the js/css are from

Author: branfosj

docs/asciinema/README.md

@@ -0,0 +1,14 @@
+# asciinema
+
+This folder contains the [asciinema](https://www.asciinema.org/) javascript and css files.
+These have been downloaded from the [asciinema release page](https://github.com/asciinema/asciinema-player/releases).
+
+## Update asciinema
+
+1. Download `asciinema-player.css` and `asciinema-player.min.js` from the [latest asciinema release}(https://github.com/asciinema/asciinema-player/releases).
+1. Rename both files to add in the version (`-VERSION`) they correspond too.
+   * `mv asciinema-player.css asciinema-player-VERSION.css`
+   * `mv asciinema-player.min.js asciinema-player-VERSION.min.js`
+1. Edit `mkdocs.yml` to update the information in `extra_css` and `extra_javascript`.
+1. Delete the old versions of these files.
+