Update readme with filters not and header

README.md

@@ -98,16 +98,17 @@ hooks:
       - secret_key_02
     filter:
       or:
-        - json:
-            pointer: /ref
-            regex: refs/heads/master
+        - not:
+            json:
+              pointer: /ref
+              regex: refs/heads/dev
         - and:
           - json:
               pointer: /ref
               regex: refs/heads/a_branch
-          - json:
-              pointer: /after
-              regex: f6e5fe4fe37df76629112d55cc210718b6a55e7e
+          - header:
+              field: X-Gitea-Event
+              regex: push
 ```
 
 ##### Command
@@ -163,10 +164,20 @@ hook should be executed.
 
 ###### Conjunction Filters
 Conjunction filters contain lists of other filters.
+- `not`: Logical negation.
 - `and`: Logical conjunction.
 - `or`: Logical disjunction.
 
 ###### Concrete Filters
+- `header`:
+
+  The `header` filter matches a regular expression on a field from the
+  received http(s) request header.
+
+  - `field`: The header field which should be matched.
+  - `regeq`: Regular expression which has to match the specified
+    header field.
+
 - `json`:
 
   The `json` filter matches a regular expression on a field from the