[#943] Add some minimal extension best practices

There's probably a lot more to mention here.

CONTRIBUTING.rst

@@ -44,6 +44,7 @@ For contributing translations to CKAN, see
 
    i18n
 
+.. _coding standards:
 
 ----------------
 Coding Standards

doc/writing-extensions.rst

@@ -547,15 +547,27 @@ Localizing extensions
 Best practices for writing extensions
 -------------------------------------
 
-.. todo:: Write up best practices for writing extensions.
-
-   * Keep your code separate from CKAN so that internal CKAN changes
-     don't break your code between releases.
-   * import style
-   * Don't import ckan, use toolkit
-   * Use the toolkit
-   * get_action()
-   * Don't edit or key to model tables
+Follow CKAN's coding standards
+==============================
+
+See :ref:`coding standards`.
+
+Use the plugins toolkit instead of importing CKAN
+=================================================
+
+Try to limit your extension to interacting with CKAN only through CKAN's
+:ref:`plugin interfaces <plugin-interfaces-reference>` and
+:ref:`plugins toolkit <plugins-toolkit>`. It's a good idea to keep your
+extension code separate from CKAN as much as possible, so that internal changes
+in CKAN from one release to the next don't break your extension.
+
+
+Don't edit CKAN's database tables
+=================================
+
+An extension can create its own tables in the CKAN database, but it should
+write to core CKAN tables directly, add columns to core tables, or use foreign
+keys against core tables.
 
 
 .. _plugin-interfaces-reference: