Document how custom tools can be created and how you can write a custom document transformation

[michael]

I'll explain this on the example of search+replace, which I implemented in the Substance Starter repo:

Transformation:
https://github.com/substance/starter/blob/master/src/transformations/search_replace.js

Tool (really good idea to have that extra thing, since we have no connection with the surface)
https://github.com/substance/starter/blob/master/src/tools/search_replace_tool.js

ToolComponent (maybe should not inherit from ToolComponent):
https://github.com/substance/starter/blob/master/src/components/search_replace_tool_component.js