Added mention to removeIndexByName()
Added mention to removeIndexByName()

Only `removeIndex()` was mentioned in the docs. 

It took me quite a while to figure out that it expected the field names (and not the index names), so I'm adding the reference.
ikagawa/phinx,Thamrongsak/phinx,robmorgan/phinx,daniel-gomes-sociomantic/phinx