List of identities not comprehensive

This is a reponse to @freakboy3742
(https://github.com/django/djangoproject.com/pull/84/files#r17401314 and
the ensuing discussion). Since it's impossible to construct a
comprehensive list of social identities, I've added the language
"includes, but is not limited to" to make it clear that we want to be
welcoming and supportive of _everyone_.

For some background, I assembled this list based on similar lists I
found in a few places, such as:

- [National Association of Social Workers code of
  ethics](https://www.socialworkers.org/pubs/code/code.asp)
- [University of Michigan Office for Institutional
  Equity](http://www.hr.umich.edu/oie/ndpolicy.html)

I then edited the list with @evildmp.
Added "educational level"

I compared our list with
http://www.ieee.org/about/corporate/governance/p9-26.html and realized
that we were missing something that addressed discrimination
based on educational level. I thought that might be something that comes
up in a technical community such as ours and so is worth including in
this list. IEEE used "matriculation" but I think "educational level,"
while longer, is less ambiguous.