Add more information regarding proposal selection, and mentor availability to GSoC "General Information" page.

[akmadian]

Is your feature request related to a problem? Please describe.
Recently in the cc-gsoc Slack channel, @kgodey provided some very useful information regarding mentor availability, and how n project ideas may be available, but there may only be enough mentors/ slots to support < n proposals. Additionally, she provided some information about proposal review criteria. She already added some of this information in #60, but I don't believe that what was added covers all of what could be useful to future participants.

Describe the solution you'd like
I believe that more available information is (in most cases) better, and I would have liked to have had this information when I started with GSoC. Because I believe that the addition of this information could help future GSoC participants, I would like to add more information regarding mentor/ slot availability to the Mentors section, and more information about the proposal review criteria to the Proposal review criteria section.

I would be happy to work with any maintainers to refine the language of the additions to be site-ready. Pending confirmation that this change is welcome, I will fork, and get to work!

Describe alternatives you've considered
I do not think there are any alternatives to just adding the information.

Additional context
Kriti's addition of proposal review criteria (PR #60)

These changes will be in ./content/gsoc-2019/contents.lr

Two recent threads in the cc-gsoc Slack channel: Thread 1 (started by Theo M.; "Does Google ultimately decide what proposals are accepted or is it entirely the organization's choice?"),
and Thread 2 (started by myself; a continutation of the replies in thread 1).

[kgodey]

@akmadian do you want to work on this?

[akmadian]

Sure! I’ll get started tonight.