table.count property, plus made a start on table documentation

docs/table.rst

@@ -0,0 +1,37 @@
+=======
+ Table
+=======
+
+Tables are accessed using the indexing operator, like so:
+
+.. code-block:: python
+
+    from sqlite_utils.db import Database
+    import sqlite3
+
+    database = Database(sqlite3.connect("my_database.db"))
+    table = database["my_table"]
+
+If the table does not yet exist, it will be created the first time you attempt to insert or upsert data into it.
+
+Introspection
+=============
+
+If you have loaded an existing table, you can use introspection to find out more about it::
+
+    >>> db["PlantType"]
+    <sqlite_utils.db.Table at 0x10f5960b8>
+    >>> db["PlantType"].count
+    3
+    >>> db["PlantType"].columns
+    [Column(cid=0, name='id', type='INTEGER', notnull=0, default_value=None, is_pk=1),
+     Column(cid=1, name='value', type='TEXT', notnull=0, default_value=None, is_pk=0)]
+    >>> db["Street_Tree_List"].count
+    189144
+    >>> db["Street_Tree_List"].foreign_keys
+    [ForeignKey(table='Street_Tree_List', column='qLegalStatus', other_table='qLegalStatus', other_column='id'),
+     ForeignKey(table='Street_Tree_List', column='qCareAssistant', other_table='qCareAssistant', other_column='id'),
+     ForeignKey(table='Street_Tree_List', column='qSiteInfo', other_table='qSiteInfo', other_column='id'),
+     ForeignKey(table='Street_Tree_List', column='qSpecies', other_table='qSpecies', other_column='id'),
+     ForeignKey(table='Street_Tree_List', column='qCaretaker', other_table='qCaretaker', other_column='id'),
+     ForeignKey(table='Street_Tree_List', column='PlantType', other_table='PlantType', other_column='id')]