Documented how to build from sources on ARM Linux.

Signed-off-by: Akos Kitta <kittaakos@typefox.io>

docs/internal/Arm.md

@@ -0,0 +1,59 @@
+### Building from the sources on Linux ARM
+
+Building the Pro IDE on Linux `armv7l` (aka `armhf`) and `aarch64` (aka `arm64`):
+
+1. Install Node.js 10.x with [nvm](https://github.com/nvm-sh/nvm#install--update-script):
+    ```
+    wget -qO- https://raw.githubusercontent.com/nvm-sh/nvm/v0.35.3/install.sh | bash
+    ```
+    Restart your shell then:
+    ```
+    nvm install 10.15.3
+    nvm use 10.15.3
+    ```
+    Verify:
+    ```
+    node -v
+    v10.15.3
+    ```
+
+2. Install [Yarn](https://classic.yarnpkg.com/en/docs/install/#debian-stable):
+
+    Configure the Debian package repository; otherwise, you will pull a different `yarn`.
+    ```
+    curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
+    echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
+    ```
+    Install:
+    ```
+    sudo apt update && sudo apt install --no-install-recommends yarn
+    ```
+    Verify:
+    ```
+    yarn -v
+    1.22.4
+    ```
+
+3. Other native [dependencies](https://github.com/eclipse-theia/theia/blob/master/doc/Developing.md#prerequisites):
+    - `make`,
+    - `gcc`,
+    - `pkg-config`,
+    - `build-essential`,
+    - `libx11-dev`, and
+    - `libxkbfile-dev`
+
+4. [Build it](https://github.com/bcmi-labs/arduino-editor#build-from-source) from the source:
+    ```
+    git clone https://github.com/bcmi-labs/arduino-editor.git \
+    && cd arduino-editor \
+    && yarn \
+    && yarn rebuild:electron \
+    && yarn --cwd ./electron-app start
+    ```
+
+5. Troubleshoot
+
+    If you see [`ENOSPC` errors](https://github.com/eclipse-theia/theia/blob/master/doc/Developing.md#linux) at runtime, increase the default `inotify` watches:
+    ```
+    echo fs.inotify.max_user_watches=524288 | sudo tee -a /etc/sysctl.conf && sudo sysctl -p
+    ```