Implementation of wcs_to_celestial_frame
Minor improvements
Make wcs_to_celestial_frame extensible
Added tests and docstring for ``wcs_to_celestial_frame``
Skip doctest
Added changelog entry
Fix parallel testing of test_earth.py
Add development/affiliated-packages.rst docs page.

At the moment this is a copy of the `package-template/README.rst` page,
but the idea is that this documentation is restructured after moving it here.
Change phrasing to clarify that it applies to Quantity not Unit

Closes #2375
Use context manager to register custom frame identifiers
Motivated by this comment
<https://github.com/astropy/astropy/pull/2835#discussion_r15990257>
this adds the make_func_with_sig function to astropy.utils.misc.
Taking advantage of this, a new astropy.utils.compat.misc.wraps
function is provided as a replacement for functools.wraps, which
provides the same functionality but while also preserving the original
wrapped function's signature (so that it displays properly in help())
Turns out there are fewer usages of functools.wraps in Astropy than I thought. And most of them are in utilties/backwards compatibility features that aren't terribly important.  But I found this one case where the new wraps implementation is useful. And there could be others in the future.
Merge pull request #2819 from mwcraig/update-quant-unit-docs

Minor documentation change to clarify Numpy functions work well with Quantity, not Unit
Enable ‘lite’ PEP8 testing
Use a registry of known formatter classes.  This has the potential advantage of enabling user-defined formatters.  astropy.units.format.__init__ should still import all the built-in formatters to ensure they are added to the registry, so this is a minimal change.
Merge pull request #2746 from astrofrog/pep8-lite

Enable ‘lite’ PEP8 testing
Added a parser parameter to HTML reader
Included a changelog entry
Parser test fails if an error occurs with html.parser
Included issue number in changelog