Log of breaking changes in 0.2 beta

[wang]

See:

[Breaking change] New changes to select, annotate, key_by interface for 0.2

https://github.com/hail-is/hail/pull/3297 This PR will change the way keys on Tables and MatrixTables are dealt with in certain functions. (The doc links will be updated once this pull request lands.) key_by(*expr, **named_exprs) (and key_rows_by and key_cols_by) is the only method that can modify key fields. The interface is identical to the current select interface, where non-named exprs must be field references (not necessarily top-level), but non-field-reference expressions can still be u…