Keep track of changes to master

There is a new file `changes_to_master.rst` that contains links like https://github.com/clawpack/pyclaw/compare/v5.1.0...master
that show changes to master since the last release, see http://clawpack.github.io/doc/changes_to_master.html

I propose using this file as follows:
When there is a new release 5.2.0:
- Copy this file to `release_5_2_0.rst` and change `v5.1.0...master` to `v5.1.0...v5.2.0` in the compare links.
- In `changes_to_master.rst`, change `v5.1.0...master` to `v5.2.0...master` in the compare links, and remove all the comments about changes in each repo.

As new changes are added to master, make comments on them in `changes_to_master.rst` if they are worth writing about, so this file will be up to date for the next release.

I also added a draft of `release_5_2_0.rst` to start writing about some changes not yet merged into master.

Somebody might want to add comments about changes to `pyclaw` to one of these files, and/or update https://github.com/clawpack/pyclaw/blob/master/CHANGES.md for the 5.2.0 release.