Docs: Set Headers

doc-src/setheaders.html

@@ -1,2 +1,18 @@
 
-Setheaders.
+This feature lets you specify a set of headers to be added to requests or
+responses, based on a filter pattern. You can specify these either on the
+command-line, or through an interactive editor in mitmproxy.
+
+<table class="table">
+    <tbody>
+        <tr>
+            <th width="20%">command-line</th>
+            <td>
+                --setheader PATTERN
+            </td>
+        </tr>
+        <tr>
+            <th>mitmproxy shortcut</th> <td><b>H</b></td>
+        </tr>
+    </tbody>
+</table>