Write documentation on plugins and how to write your own

[Gaya]

Thyme has plugin functionality, but has no documentation on it.

Write about:

• registerSettingsPanel, registerInjectable, registerTableColumn (to be changed!)
• different places you can register injectables to right now (and update thyme-connect urls to docs)
• how to write your own plugin
• write about how to hook into reducers and add more fields
• explain how to register your own epics
• document registerable reducer places