Put items in issue-order in CHANGELOG.  Also correct one item for consistency
(#526)
Have addNoiseSNR tests use default constructed Noise object, since that's where we initially had trouble.
(#526)
Various (I believe) uncontroversial edits to the documentation
(#526)
Remove backticks around math that isn't really code
(#526)
Remove reference to non-existent (in GalSim repo at least) precess.f file.
(#526)