[community] suggested glossary additions

Hi Community Team,

Here are some of my suggested additions to the glossary page at https://trac.torproject.org/projects/tor/wiki/doc/community/glossary that could be helpful to new contributors (or maybe just people interested in the project):

=== GSoC === Tor participates in the Google Summer of Code, which is a summer program for university students.

=== TPI === TPI is an acronym for The Tor Project, Inc.

=== tpo === People on IRC often use `tpo` to abbreviate `torproject.org` when writing hostnames. For example, `trac.tpo` is an abbreviation for `trac.torproject.org`.