Fix grammar and punctuation in README.md

I enjoy using immutable a lot and wanted to contribute.

I found the expression in Line 333 of the README.md to be unclear when reading the documentation, so I made the following change:

BEFORE:
There is a performance penalty paid every time you create a new immutable object via applying a mutation.

AFTER:
Applying a mutation to create a new immutable object will result in a performance penalty.

I also added a comma after "a series of mutations" on line 334.

Nothing else was changed besides above.