Add community section

I'd like to propose the following community section in the docs that contains links to resources made by the Cloud Foundry community.

In specific; please note the link to [CF Contrib](https://github.com/mrdavidlaing/cf-docs/pull/new/cloudfoundry:master...mrdavidlaing:add_community_section#L1R5) - an "unofficial" site listing bits and pieces made available & curated by the CF Community.