Add CONTRIBUTING.md

Author: mumoshu

CONTRIBUTING.md

@@ -0,0 +1,31 @@
+Before submitting a pul request, I'd encourage you to test it yourself.
+
+To do so, you need to run the plugin indirectly or directly-
+
+**Indirect** Install the plugin locally and run it via helm:
+
+```
+$ helm plugin uninstall diff
+
+$ helm plugin list
+#=> Make sure that the previous installation of helm-diff has unisntalled
+
+$ make install
+
+$ helm plugin list
+#=> Make sure that the version of helm-diff built from your branch has instaled
+
+$ helm diff upgrade ... (snip)
+```
+
+**Direct** Build the plugin binary and execute it with a few helm-specific environment variables:
+
+```
+$ go build .
+
+$ HELM_NAMESPACE=default \
+HELM_BIN=helm372 \
+  ./helm-diff upgrade foo $CHART \
+  --set argo-cd.nameOverride=testtest \
+  --install
+```