Add Prettier.md to document compatibility issues (fixes #410).

doc/Prettier.md

@@ -0,0 +1,23 @@
+# Using markdownlint with Prettier
+
+[`Prettier`](https://prettier.io) is a popular code formatter. For the most part,
+it works seamlessly with `markdownlint`. Special situations are documented
+below.
+
+## List item indentation
+
+The default settings of `markdownlint` and `Prettier` are compatible and don't
+result in any linting violations. If `Prettier` is used with `--tab-width` set
+to `4`, the following `markdownlint` configuration can be used:
+
+```json
+{
+  "MD007": {
+    "indent": 4
+  },
+  "MD030": {
+    "ul_single": 3,
+    "ul_multi": 3
+  }
+}
+```