[Dev] Adds .devcontainer manifest to help promote contributions (#342)

* Added a devcontainer file

* linting

* Added link to README

.devcontainer/devcontainer.json

@@ -0,0 +1,12 @@
+{
+  "name": "Pinchflat Dev",
+  "dockerComposeFile": "../docker-compose.yml",
+  "service": "phx",
+  "workspaceFolder": "/app",
+  "shutdownAction": "stopCompose",
+  "customizations": {
+    "vscode": {
+      "extensions": ["phoenixframework.phoenix", "JakeBecker.elixir-ls", "esbenp.prettier-vscode"]
+    }
+  }
+}