Create pxtCPPbuild.md

Author: DeveloperTryingToCodeLikeOtherOfThem

docs/install/pxtCPPbuild.md

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+## PXT C++ Build
+To build and install the PXT C++ build, you first have to install Node.js.
+``` bash
+npm install
+npm run build
+```
+
+Then, install the `pxt` command line tool (**only once without clicking anything**).
+``` bash
+npm install -g pxt
+```
+
+After that install `gulp` (**only once without click anything**).
+``` bash
+npm install -g gulp
+```
+Finally, install PXT and C++ build.
+``` bash
+  pxt add cpp
+  pxt build
+  ```