Add contributing tips for dzil and git

[yanick]

Tim proposed that information on how to deal with dzil should be included to a CONTRIBUTING file (or contrib section of the docs).

Excerpt of the discussion with pertinent info:

[10:13] <yanick> So if you want to do things and be able to test and build, you have three choices:
[10:15] <yanick> (a) do dzil authordeps | cpanm, go get a coffee, a solid breakfast, and a nap (I'm using a lot of dzil plugins, so the dep chain is long), and come back to be able to do 'dzil build' whenever you want. 
[10:15] <yanick> (b) I think that if you add a $VERSION = '1.99'; # whatever version will work to DBD::Oracle, it'll make the Bootstrapper happy. 
[10:16] <yanick> (c) You can work on the releases branch, and leave to me the merging back to 'master'. 

Additional note: also explain what is 'master', and what is 'releases'.

[yanick]

Now that the version is auto-filled, that's not as needed as it was. Still, a small guide would still be usefull