docs: improve several plugin READMEs and add one for ssh plugin

plugins/ssh/README.md

@@ -0,0 +1,16 @@
+# ssh plugin
+
+This plugin provides host completion based off of your `~/.ssh/config` file, and adds
+some utility functions to work with SSH keys.
+
+To use it, add `ssh` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... ssh)
+```
+
+## Functions
+
+- `ssh_rmhkey`: remove host key from known hosts based on a host section name from `.ssh/config`.
+- `ssh_load_key`: load SSH key into agent.
+- `ssh_unload_key`: remove SSH key from agent.