Add entities-reducer

[kwelch]

• Don't create a pull request to add a library, instead use devarchy.com/redux.
• Only create a pull request to change the categorization.
• Open an issue if you are not sure how to change the categorization to fit a library you want to add.

[brillout]

Is there a reason you didn't use https://devarchy.com/redux to add it?

[kwelch]

That was my fault. I didn't read.