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

plugins/mosh/README.md

+# 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)
+```

plugins/themes/README.md

+# Themes Plugin
+
+This plugin allows you to change ZSH theme on the go.
+
+To use it, add `themes` to the plugins array in your zshrc file:
+
+```
+plugins=(... themes)
+```
+
+## Usage
+
+`theme <theme_name>` - Changes the ZSH theme to specified theme.
+
+`theme ` - Changes the ZSH theme to some random theme.
+
+`lstheme ` - Lists installed ZSH themes.
+

plugins/yum/README.md

+# Yum plugin
+
+This plugin adds useful aliases for common [Yum](http://yum.baseurl.org/) commands.
+
+To use it, add `yum` to the plugins array in your zshrc file:
+
+```
+plugins=(... yum)
+```
+
+## Aliases
+
+| Alias | Command                           | Description                  |
+|-------|-----------------------------------|------------------------------|
+| ys    | `yum search`                      | Search package               |
+| yp    | `yum info`                        | Show package info            |
+| yl    | `yum list`                        | List packages                |
+| ygl   | `yum grouplist`                   | List package groups          |
+| yli   | `yum list installed`              | Print all installed packages |
+| ymc   | `yum makecache`                   | Rebuild the yum package list |
+| yu    | `sudo yum update`                 | Upgrade packages             |
+| yi    | `sudo yum install`                | Install package              |
+| ygi   | `sudo yum groupinstall`           | Install package group        |
+| yr    | `sudo yum remove`                 | Remove package               |
+| ygr   | `sudo yum groupremove`            | Remove pagage group          |
+| yrl   | `sudo yum remove --remove-leaves` | Remove package and leaves    |
+| yc    | `sudo yum clean all`              | Clean yum cache              |
+