Document npm module install dependencies

doc/npm.md

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+# npm Install Requirements
+
+If you're installing the npm module you'll need certain dependencies to build
+the native modules used by VS Code.
+
+## Ubuntu, Debian
+
+```bash
+sudo apt-get install -y \
+  build-essential \
+  pkg-config \
+  libx11-dev \
+  libxkbfile-dev \
+  libsecret-1-dev
+```
+
+## Fedora, Red Hat, SUSE
+
+```bash
+sudo yum groupinstall -y 'Development Tools'
+sudo yum config-manager --set-enabled PowerTools
+sudo yum install -y python2 libsecret-devel libX11-devel libxkbfile-devel
+npm config set python python2
+```
+
+## macOS
+
+Install [Xcode](https://developer.apple.com/xcode/downloads/) and run:
+
+```bash
+xcode-select --install
+```