Add plugin template (#738)

Adds a new plugin template to the `init` command: `rojo init --kind
plugin`.

assets/default-plugin-project/README.md

@@ -0,0 +1,17 @@
+# {project_name}
+Generated by [Rojo](https://github.com/rojo-rbx/rojo) {rojo_version}.
+
+## Getting Started
+To build this plugin to your local plugins folder, use:
+
+```bash
+rojo build -p "{project_name}.rbxm"
+```
+
+You can include the `watch` flag to re-build it on save:
+
+```bash
+rojo build -p "{project_name}.rbxm" --watch
+```
+
+For more help, check out [the Rojo documentation](https://rojo.space/docs).