Add a stub for incremental algorithms

Author: hyanwong

_toc.yml

@@ -12,6 +12,7 @@ parts:
   chapters:
   - file: analysing_tree_sequences
   - file: analysing_trees
+  - file: incremental_algorithms
   - file: counting_topologies
   - file: parallelization
 - caption: Further tskit tutorials

incremental_algorithms.md

@@ -0,0 +1,32 @@
+---
+jupytext:
+  text_representation:
+    extension: .md
+    format_name: myst
+    format_version: 0.12
+    jupytext_version: 1.9.1
+kernelspec:
+  display_name: Python 3
+  language: python
+  name: python3
+---
+
+```{currentmodule} tskit
+```
+
+(sec_incremental)=
+# _Incremental algorithms_
+
+Much of the [efficiency](sec_what_is_analysis)
+of the tskit approach comes from the use of incremental algorithms.
+By considering only the difference between adjacent trees,
+incremental algorithms avoid having to perform the same
+calculation multiple times on different trees.
+
+This tutorial will explain the philosophy behind incremental algorithms,
+and provide examples of how to create your own (e.g. using the
+{meth}`TreeSequence.edge_diffs` method).
+
+:::{todo}
+Create content. See [issue 233](https://github.com/tskit-dev/tutorials/issues/233)
+:::