Update docs to fix installation procedure errors; grammar fixes

* Grammar fixes in README, manager.md
* README points to wrong url for issue tracker
* Add missing dev-master in composer require command
* Add missing underscore in bundle config
* Add instructions for updating Doctrine resolve_target_entities config
* Add missing end-of-block markup so to link to the PaymentManager API works
Merge pull request #1 from jbafford/docs-fixes

Update docs to fix installation procedure errors; grammar fixes