Create an index/resources section in the readme

[taladrane]

Finding the things that we've worked on or are interested in can be harder than expected given the way the Foundation is organized in GitHub, i.e. we often look in this repo to find the guides we've written, which live elsewhere. We should add an index to the repo (read me? new resources doc?) that links to vulnerability-related resources that we've either helped develop or refer to extensively.

I have had trouble locating stuff like this before. I suggest we add an index to document where these things live because it's not consistent
Originally posted by @ctcpip in #150 (comment)