Added README to contrib folder for Snorkel DB backend

metal/contrib/backends/README.md

@@ -0,0 +1,14 @@
+# Backends
+
+### Snorkel Databases
+This provides a generic wrapper class for using Snorkel v0.7 databases in MeTal.  We make several assumptions when using this class:
+- The snorkel database is fully initialized (e.g., candidates, splits, probabilistic and gold labels are pre-defined).
+- The application is a standard unary/binary relation extraction task.
+- The dataset fits in local memory. 
+
+Usage example:
+```
+db_conn_str = "cdr.db"
+candidate_def = ['ChemicalDisease', ['chemical', 'disease']]
+train, dev, test = SnorkelDataset.splits(db_conn_str, candidate_def)
+```