Add handler docs

README.md

@@ -10,6 +10,7 @@ A customizable Neovim plugin to run code inside the editor
 - [Installation](#installation)
 - [Usage](#usage)
 - [Configuration](#configuration)
+- [Handlers](#handlers)
 - [Helpers](#helpers)
 - [Advanced handlers configurations](#advanced-handlers-configurations)
 - [Contribution](#contribution)
@@ -53,7 +54,10 @@ A customizable Neovim plugin to run code inside the editor
 ## Usage
 
   ```lua
-  require('runner').run() -- Can also be called with the buffer number where the code is
+  require('runner').run()
+
+  -- Can also be called with the buffer number where the code is:
+  -- require('runner').run( <buffer_number> )
 
   -- To set a mapping
   vim.keymap.set('n', '<leader><space>', require('runner').run)
@@ -63,7 +67,8 @@ A customizable Neovim plugin to run code inside the editor
 
   #### `setup(options)`
 
-  Runner comes with the following defaults:
+  Using this setup function is **optional**. Runner comes with the following defaults:
+
   ```lua
   require('runner').setup({
     position = 'right', -- position of the terminal window when using the shell_handler
@@ -74,9 +79,23 @@ A customizable Neovim plugin to run code inside the editor
   })
   ```
 
+## Handlers
+
+  Runner has the concept of handlers. Handlers are language specific and are little
+  functions that run code. For example, a handler could run the project, run the tests,
+  build the project, or even run custom project scripts.
+
+  A handler is a function that accepts a buffer number as an argument, and doesn't
+  return anything. This buffer number is the buffer where the code to run is located.
+  It defaults to the buffer where the `run()` function was called, or, if specified,
+  is the buffer number that was passed to the `run()` function. Runner runs the
+  appropriate handler based on this buffer's filetype.
+
+  For using multiple handlers on the same filetype, see the [choice helper](#choice(handlers)).
+  
   #### `set_handler(filetype, handler)`
 
-  Default handlers can be found [here](./lua/runner/handlers/init.lua).
+  This function **overwrites** the handler set for the specified filetype. Default handlers can be found [here](./lua/runner/handlers/init.lua).
 
   | Argument name | Description | Type |
   |---------------- | --------------- | --------------- |
@@ -111,6 +130,7 @@ A customizable Neovim plugin to run code inside the editor
     - `select_horizontal` (defaults to `<C-X>`): Opens the window at the bottom of the screen.
     - `select_vertical` (defaults to `<C-V>`): Opens the window at the right of the screen.
     <br/>
+    <br/>
 
     | Argument name | Description | Type |
     |---------------- | --------------- | --------------- |