Added PIO recommendation when opening in vscode

.gitignore

@@ -2,3 +2,4 @@
 .pioenvs
 .piolibdeps
 .vscode
+!.vscode/extensions.json

.vscode/extensions.json

@@ -0,0 +1,7 @@
+{
+	// See http://go.microsoft.com/fwlink/?LinkId=827846
+	// for the documentation about the extensions.json format
+	"recommendations": [
+		"platformio.platformio-ide"
+	]
+}