Add a guide to the plugin authors section about adding support for new chains with existing Chain Types

[fvictorio]

A plugin that adds built-in support for a chain is relatively straightforward: it just needs to extend the user config to include an extra chain descriptor.

We should have a guide that explains how to do this. Later we could also have a plugin template for this, but that's out of scope for this issue.

[alcuadrado]

Note: this is for chains that use existing Chain Types