Add more docs for Feature Layer

not complete, but progress..

docs/features/transactions/index.rst

@@ -0,0 +1,17 @@
+
+POS Transactions
+================
+
+Presumably all retailers have a dedicated point of sale (POS) system
+which is where most if not all transactions occur.
+
+Rattail is not and likely will never be a POS system, so it's not
+concerned with faciliating the transaction, but rather in "harvesting"
+its data for whatever purposes you may have.
+
+
+.. toctree::
+   :maxdepth: 1
+
+   importing/index
+   reporting/index