add a note about forking the repo in README.md (#52)

.gitignore

@@ -28,3 +28,4 @@ whitelist.json
 src/main/resources/mixins.*.json
 *.bat
 layout.json
+.DS_Store

README.md

@@ -0,0 +1,13 @@
+# Waila
+
+## Contribution
+
+When forking this repository, make sure to uncheck the `Copy the master branch only` option.
+This will copy all tags to your fork, so the build system can detect the latest mod version.
+If you leave it checked, you won't be able to enter the world in a local client run because Waila [will think the mod is outdated](https://github.com/GTNewHorizons/waila/blob/master/src/main/java/mcp/mobius/waila/Waila.java#L133).
+
+If you already forked the repository without tags, you can fetch them manually:
+```
+git remote add upstream https://github.com/GTNewHorizons/waila
+git fetch upstream --tags
+```