ship: talk about Macro Development

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@@ -30,3 +30,25 @@
   Undo `make install` with
 
     make distclean
+
+## Macro Development
+
+  To define a new macro, you have to add a function like
+
+    ## usage: BRE -> FUNCTION_NAME \1 [\2 ...]
+
+  where `BRE` is a basic regular expression, that has to match a whole
+  line.  `FUNCTION_NAME` should be the name of a function that outputs
+  a `sed` script.  `\1` refers to the line number, where the macro is
+  used and `\2 ...` are the backreferences into `BRE`.  E.g.
+
+    ## usage: #@date \([.*]\) -> build_datemacro \1 \2
+    build_datemacro() {
+      printf '%da\\\n%s\n' "$1" "$(date +"$2")"
+    }
+
+  Like in this example, the line number `\1`, which gets mapped to `$1`,
+  is usually used to only change the line, where the macro got called.
+  The second argument gets passed as format specifier to `date`.
+
+  Further examples can be found in `./build`.