[Doc] Add note to front README about format stability
[Doc] Add note to front README about format stability

Close #183

Author: Wes McKinney <wesm@apache.org>

Closes #184 from wesm/doc/readme-stability and squashes the following commits:

3e8d68d [Wes McKinney] Add note to README about format stability

wesm/feather,wesm/feather,wesm/feather,wesm/feather,wesm/feather,wesm/feather,wesm/feather,wesm/feather