doc: add backword_compatibility.md

@barakmich @jonboulle @philips @kelseyhightower 
Caps, phrasing...

"Default data dir location has changed from..."
"The disk format within the data dir has changed. etcd 0.5 should be able to auto upgrade the old data format. Instructions on doing so manually are in the [migration tool doc][migrationtooldoc]

[migrationtooldoc]: https://github.com/coreos/etcd/blob/master/Documentation/0.5/0_4_migration_tool.md