Document solvable blocks

[qtothec]

Per @whart222 in #94, on the implementation of solvable blocks:

I want to re-open this issue. This is a big enough change that I would expect some documentation of it, other than the changes in the test directory.

We haven't historically done this documentation, and I'm open to suggestions. But what about having a blog-like documentation of the CHANGELOG? This would contain significant changes to master. If the online documentation has been updated, then the blog could simply reference those updates. But in a case like this, where the design might change, the blog would contain the description of this change.

Making this a distinct issue to improve glanceability from issues list and IDAES project board.