docs: CONTRIBUTING.md: clarify what should be generated, using official colours and add a TOC (#630)

Author: alex-courtis

CONTRIBUTING.md

@@ -2,6 +2,28 @@
 
 Thank you for your contribution!
 
+<!-- 
+https://github.com/jonschlinkert/markdown-toc
+markdown-toc --maxdepth=3 -i CONTRIBUTING.md
+-->
+
+<!-- toc -->
+
+- [Prerequisites](#prerequisites)
+  * [stylua](#stylua)
+  * [luacheck](#luacheck)
+- [Adding icons](#adding-icons)
+- [Building](#building)
+  * [Generate](#generate)
+  * [Validate](#validate)
+- [Test](#test)
+- [Documentation](#documentation)
+- [Pull Request](#pull-request)
+- [Subject](#subject)
+- [Browser Font](#browser-font)
+
+<!-- tocstop -->
+
 ## Prerequisites
 
 Code is formatted using *stylua* and linted using *luacheck*.
@@ -40,11 +62,11 @@ If you experience `luarocks` dependency issues installing `luacheck`, you may ne
 luarocks config --local lua_version 5.1
 ```
 
-## Adding icons
+## Adding Or Updating Icons
 
-Add or update icons in `lua/nvim-web-devicons/default/` directory
+Add or update icons in `lua/nvim-web-devicons/default/` directory. Build after making your changes.
 
-There are five files where icons can be added:
+Icons must be added to `/default/` files only. The `/light/` files are automatically generated and should not be modified.
 
 1. `icons_by_filename.lua`
 2. `icons_by_file_extension.lua`
@@ -69,8 +91,9 @@ Each icon must have the following structure (this is an example):
 
 - `icon` glyph
 - `color` must contain a color in the html notation
-- `cterm_color` must contain a number (any number)
-  - the correct value for `cterm_color` will be generated by the script in next step
+  - it is desirable to use the "official" icon color: look at the website for a style guide or pick the colour from a logo
+- `cterm_color` must contain a placeholder (any number)
+  - the value for `cterm_color` will be generated by the script in next step
 - `name` must only contain alphanumeric characters (don't use `/`, `-`, `_`)
 
 ## Building