Add READMEs to plugins: yum, mosh, themes (#7229)

plugins/mosh/README.md

@@ -0,0 +1,9 @@
+# Mosh Plugin
+
+This plugin allows SSH tab completion for [mosh](https://mosh.org/) hostnames.
+
+To use it, add `mosh` to the plugins array in your zshrc file:
+
+```
+plugins=(... mosh)
+```