add luacheck badge to readme / execute luacheck on PR's

.github/workflows/luacheck.yml

@@ -1,6 +1,6 @@
 name: luacheck
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build:

README.md

@@ -2,6 +2,8 @@
 
 # scifi_nodes
 
+![](https://github.com/D00Med/scifi_nodes/workflows/luacheck/badge.svg)
+
 Minetest mod that adds scifi themed blocks, doors, materials, plants and other assets.
 
 # Changes log