Add Set Inclusion With hstore as a postgres til.

postgres/set-inclusion-with-hstore.md

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+# Set Inclusion With hstore
+
+In PostgreSQL, `hstore` records can be compared via set inclusion. The `@>`
+and `<@` operators can be used for this. The `@>` operator checks if the
+right operand is a subset of the left operand. The `<@` operator checks if
+the left operand is a subset of the right operand.
+
+```sql
+> select '"one"=>"1", "two"=>"2", "three"=>"3"'::hstore @> '"two"=>"2"'::hstore;
+ ?column?
+ ----------
+  t
+
+> select '"one"=>"1", "two"=>"2", "three"=>"3"'::hstore <@ '"two"=>"2"'::hstore;
+ ?column?
+----------
+ f
+```
+
+See the [`hstore` PostgreSQL
+docs](http://www.postgresql.org/docs/current/static/hstore.html) for more
+details.