README updates

- Mention more infrequent changes
- Add CONTRIBUTING.md
- Mention maintainership in CONTRIBUTING.md

Merge pull request go-gorp#219 from sqdk/sqdk-patch-2-warn

Update gorp.go

columnToFieldIndex: revert b5ce3b9 change, as it breaks tag mapping w…

…hen a field marked transient matches a column mapped to another field. Fixes go-gorp#175 but reverses go-gorp#146

TestSelectAlias: modify to demonstrate how this behavior can be achieved using two separate structs - one that reflects the actual table structure (for inserts/updates) and one with the extended field, which can be used for selects.

v1.2