Add upgrading docs

This is an initial draft of docs for converting your packages. Initially i'm thinking I want to have 3 docs

* Converting your package
* Converting your UI theme
* Converting your Syntax theme

My vision for them is written around specific patterns with short explanations and links to examples. Like "I have a SelectListView view, how do I do this with the new APIs?". And we can link to our [PRs for conversion](https://github.com/atom/command-palette/pull/19/files). Deprecations will get all the nits, but there are definite patterns we will find. 

We can really flesh these out as we convert packages. Would be awesome for all y'all to pitch in :)