Add basic docs for "receiving" feature

docs/features/purchasing/index.rst

@@ -0,0 +1,17 @@
+
+Purchasing
+==========
+
+Presumably all retailers purchase products from vendors.
+
+Rattail offers tools to assist with the various workflows involved.
+It splits the life cycle into 3 "phases" - although some purchases may
+not explicitly go through every phase.  In particular it's possible
+for the Receiving phase to include everything Accounting does.
+
+.. toctree::
+   :maxdepth: 1
+
+   ordering/index
+   receiving/index
+   accounting/index