Start a glossary of terms

docs/source/glossary.rst

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+.. _glossary:
+
+================================================
+ Glossary of Terms Used in Flake8 Documentation
+================================================
+
+.. glossary::
+    :sorted:
+
+    formatter
+        A :term:`plugin` that augments the output of |Flake8| when passed
+        to :option:`flake8 --format`.
+
+    plugin
+        A package that is typically installed from PyPI to augment the
+        behaviour of |Flake8| either through adding one or more additional
+        :term:`check`\ s or providing additional :term:`formatter`\ s.
+
+    check
+        A piece of logic that corresponds to an error code. A check may
+        be a style check (e.g., check the length of a given line against
+        the user configured maximum) or a lint check (e.g., checking for
+        unused imports) or some other check as defined by a plugin.
+
+    error code
+        The symbol associated with a specific :term:`check`. For example,
+        pycodestyle implements :term:`check`\ s that look for whitespace
+        around binary operators and will either return an error code of
+        W503 or W504.