Write down how to build C++ locally (#6569)

docs/developer/local-cpp.md

@@ -0,0 +1,49 @@
+# Local C++ development setup
+
+Install ARM GCC https://developer.arm.com/downloads/-/gnu-rm 10.3
+
+```bash
+npm install -g pxt # cmd line tool
+cd pxt-arcade
+npm install
+pxt link ../pxt
+pxt link ../pxt-common-packages
+pxt serve --rebundle # and stop it
+mkdir -p projects/myprj
+cd projects/myprj
+```
+
+Create the following files (replacing with rp2040 with the target you need):
+
+`pxt.json`
+
+```json
+{
+    "name": "myprj",
+    "version": "0.0.0",
+    "dependencies": {
+        "hw":  "file:../../libs/hw---rp2040",
+        "device": "file:../../libs/device"
+    },
+    "files": [
+        "main.ts"
+    ],
+    "supportedTargets": [
+        "arcade"
+    ]
+}
+```
+
+`main.ts`
+
+```typescript
+let mySprite = sprites.create(sprites.duck.duck1)
+mySprite.ay = 50
+mySprite.setStayInScreen(true)
+```
+
+To build the native binary:
+
+```bash
+PXT_FORCE_LOCAL=yes PXT_NODOCKER=yes PXT_RUNTIME_DEV=yes PXT_ASMDEBUG=yes pxt build
+```