Add constants layer description

docs/layers/constants.md

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+# Constatnts
+
+This layer is responsible for setting constant virables, such as action type names for redux actions. Actions must have a type property that indicates the type of action being performed. Types should typically be defined as string constants.

docs/layers/tests.md

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+# Tests
+
+This layer is responsible for configuring of redux store, such as setting reducers and midelwares. A [store](http://redux.js.org/docs/api/Store.html) holds the whole state tree of your application.