Create tithing page

[s-ben]

We need a page explaining the tithing model, with instructions on how to implement/tweak it.

I suggest a page under 'How to Use SourceCred in Your Community' (could also go in other high-level section, open to ideas). Page could be titled 'Tithing', or 'Supporting SourceCred'.

The page should give a high-level overview of what tithing is, how it supports us, how it's implemented currently (it's actually 5% of Cred, which roughly translates to 5% of Grain (sometimes that can vary too much, and we're looking at improving that btw), how to change the tithe %, that it's OK to change the %, etc.