[community] suggested glossary additions

[Taylor Yu]

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`.