rewrite to python: add section about adding marker sets

README.md

@@ -5,7 +5,7 @@ Mapping is my passionfruit!
 ## What is this?
 
 This project is meant to let PVV members contribute map data for PVV's minecraft server, like coordinates for bases, cities, and other points of interest.
-These markers are written in python so that we easily can do things like generating points in a loop, or other types of automation.
+These markers are written in python so that we easily can do things like generating points in a loop, calculating distances, and other types of automation.
 
 There is also a cli tool that we use to export these points into the formats of several popular minecraft map implementations.
 
@@ -110,7 +110,33 @@ MARKERS = [
 
 ## How to add a new marker set
 
-TODO: Write this section
+To add a new marker set, you need to:
+
+1. Create a new directory alongside the other marker sets in `src/minecraft_kartverket/marker_sets/<world>`.
+2. Create an `__init__.py` file in this directory with the following content:
+
+```python
+from minecraft_kartverket.lib_marker.marker_set import MarkerSet
+
+MARKER_SET = MarkerSet(
+    name="My Marker Set",
+    markers=[
+    ],
+)
+```
+
+3. Add the marker set to the `MARKER_SETS` list in `src/minecraft_kartverket/marker_sets/<world>/__init__.py`.
+
+```python
+from .<marker_set_name> import MARKER_SET as <marker_set_name>_marker_set
+
+MARKER_SETS = [
+    ...
+    portals_marker_set,
+]
+```
+
+See the other marker sets for inspiration.
 
 ## How to add icons