Updating links/wording (#556)
Updating links/wording (#556)

Update CONTRIBUTING.md
* Updating links/wording
* adding "becoming a maintainer"
* Updated section about branches.

burnbright/silverstripe-shop,burnbright/silverstripe-shop,burnbright/silverstripe-shop,hpeide/silverstripe-shop,silvershop/silvershop-core