Added MIGRATION.md

MIGRATION.md

@@ -0,0 +1,63 @@
+## Migration
+
+This document contains information about how to smoothly
+migrate the things from a version to another version.
+
+
+### `1.x.x` to `2.x.x`
+
+The big change is the the data schema -- which should be migrated.
+
+The old `~/.git-stats` format was:
+
+```json
+{
+   "<date>": {
+      "<remote-url>": {
+          "<hash>": "<date>"
+      }
+   }
+}
+```
+
+In the new version, the remote url is not mandatory anymore. The new format is:
+
+```json
+{
+    "commits": {
+        "<date>": {
+            "<hash>": 1
+        }
+    }
+}
+```
+
+This is supposed to change when users install the `2.x.x` versions. However, if
+the migration script fails, you can always run it manually:
+
+```sh
+./scripts/migration/2.0.0.js
+```
+
+This will modify the `~/.git-stats` file.
+
+When using `git-stats` as library, things changed too. The old way was:
+
+```js
+var GitStats = require("git-stats");
+
+GitStats.ansiCalendar(opts, fn);
+```
+
+In `2.x.x` GitStats is a constructor. That allows us to create as many `git-stats`
+instances we want.
+
+```js
+var GitStats = require("git-stats");
+
+// Provide a custom data path
+var gs1 = new GitStats("path/to/some/data.json");
+
+// Use the default (~/.git-stats)
+var gs2 = new GitStats();
+```