Merge pull request #1557 from cburgdorf/christoph/docs/clarify-local-plugin-install

Clarify local plugin install

Author: carver

docs/guides/trinity/writing_plugins.rst

@@ -332,4 +332,10 @@ Check out the `official documentation on entry points <https://packaging.python.
 for a deeper explanation.
 
 A plugin where the ``setup.py`` file is configured as described can be installed by
-``pip install <package-name>``` and immediately becomes available as a plugin in Trinity.
+``pip install <package-name>``` and immediately becomes available as a plugin in Trinity.
+
+.. note::
+
+  Plugins installed from a local directory (instead of the pypi registry), such as the sample
+  plugin described in this article, must be installed with the ``-e`` parameter (Example:
+  ``pip install -e ./trinity-external-plugins/examples/peer_count_reporter``)