Firewall / categories: add basic usage documentation. for https://github.com/opnsense/docs/issues/301

source/firewall.rst

@@ -17,11 +17,12 @@ These are all combined in the firewall section.
    :titlesonly:
 
    manual/aliases
-   manual/firewall
+   manual/firewall_categories
+   manual/firewall_groups
    manual/nat
    manual/nptv6
+   manual/firewall
    manual/shaping
-   manual/firewall_groups
    manual/firewall_settings
    manual/firewall_scrub
    manual/how-tos/carp

source/manual/firewall_categories.rst

@@ -0,0 +1,12 @@
+===========================
+Categories
+===========================
+
+To ease maintenance of larger rulesets, OPNsense includes categories for the firewall.
+Each rule can contain one or more categories, which can be filtered on top of each firewall rule page.
+
+New categories can be created from within the rule or you can use the category editor in :menuselection:`Firewall -> Categories`
+to manage them.
+
+There aren't a lot of properties available on a category, you can optionally set a color and when created from a firewall
+rule the item can have an "automatic" checkbox, in which case it will be removed automatically when unused.