Add documentation for override regexp facts

Author: kpaulisse

doc/advanced-override-facts.md

@@ -65,3 +65,16 @@ The following data types in parentheses are supported:
 | `(json)` | Treat the input as a JSON string (calls `JSON.parse` in ruby) |
 | `(boolean)` | Treat the input as a boolean -- it must be `true` or `false`, case-insensitive |
 | `(nil)` | Ignore any characters after `(nil)` and deletes the fact if the fact exists |
+
+## Regular expressions
+
+If you wish to match multiple facts by pattern, specify the regular expression in place of the key name. For example:
+
+```
+octocatalog-diff -n some-node.example.com -f master -t master \
+  --to-fact-override /^ipaddress/=10.11.12.13
+```
+
+In this example, `$::ipaddress`, `$::ipaddress_eth0`, `$::ipaddress_bond0`, and any other facts starting with "ipaddress" would be overridden. However, a fact named `$::additional_ipaddress` would not be overridden, because it does not match the regular expression.
+
+Please note that you cannot *add* a fact with a regular expression -- when using regular expressions you can only modify or delete facts.