Add instructions for creating plugins

HOW_TO_PLUGIN.md

@@ -0,0 +1,105 @@
+# How to create Tmux plugins
+
+Creating a new plugin is easy.
+
+For demonstration purposes we'll create a simple plugin that lists all
+installed TPM plugins. Yes, a plugin that lists plugins :) We'll bind that to
+`prefix + T`.
+
+### 1. create a new git project
+
+TPM depends on git for downloading and updating plugins.
+
+To create a new git project:
+
+    $ mkdir tmux_my_plugin
+    $ cd tmux_my_plugin
+    $ git init
+
+### 2. create a `*.tmux` plugin run file
+
+When it sources a plugin, TPM executes all `*.tmux` files in your plugins'
+directory. That's how plugins are run.
+
+Create a plugin run file in plugin directory:
+
+    $ touch my_plugin.tmux
+    $ chmod +x my_plugin.tmux
+
+You can have more than one `*.tmux` file, and all will get executed. Usually
+however, you'll need just one.
+
+### 3. create a plugin key binding
+
+We want the behavior of the plugin to trigger when a user hits `prefix + T`.
+
+Key `T` is chosen:
+ - it's kind of a mnemonic for `TPM`
+ - the key is not used by Tmux natively. Tmux man page, KEY BINDINGS section
+   contains a list of all the bindings Tmux uses. We don't want to override a
+   Tmux default, and there's plenty of unused keys.
+
+Open the plugin run file in your favorite text editor:
+
+    $ vim my_plugin.tmux
+    # or
+    $ subl my_plugin.tmux
+
+Put the following content in the file:
+
+    #!/usr/env/bin bash
+
+    CURRENT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+    tmux bind-key T run-shell "$CURRENT_DIR/scripts/tmux_list_plugins.sh"
+
+As you can see, plugin run file is a simple bash script that sets up binding.
+
+When pressed, `prefix + T` will now execute another shell script:
+`tmux_list_plugins.sh`. That script should be in `scripts/` directory -
+relative to the plugin run file.
+
+
+### 4. create a script that does the job
+
+Now that we have the binding, let's create a script that's invoked on
+`prefix + T`.
+
+    $ mkdir scripts
+    $ touch scripts/tmux_list_plugins.sh
+    $ chmod +x scripts/tmux_list_plugins.sh
+
+And here's the script content:
+
+    #!/usr/env/bin bash
+
+    # fetching the value of "tpm_plugins" option
+    option_value=$(tmux show-option -gqv "@tpm_plugins")
+
+    # displaying variable content
+    echo $option_value
+
+### 5. try it out
+
+To try if this works, execute the plugin run file:
+
+    $ ./my_plugin.tmux
+
+That should set up the key binding. Now hit `prefix + T` and see if it works.
+
+### 6. publish the plugin
+
+When everything works, push the plugin to an online git repository, preferably
+Github.
+
+Other users can install your plugin by just adding plugin git URL to the
+`@tpm_plugins` list in their `.tmux.conf`.
+
+If the plugin is on Github, your users will be able to use the shorthand of
+`github_username/repository`.
+
+### Conclusion
+
+Hopefully, that was easy. As you can see, it's mostly shell scripting.
+
+You can use other scripting languages (ruby, phyton etc), but plain old shell
+is preferred because it will work almost anywhere.

README.md

@@ -66,6 +66,8 @@ list.
 
 ### Creating plugins
 
+[How to create a plugin](HOW_TO_PLUGIN.md). It's easy.
+
 ### License
 
 [MIT](LICENSE.md)