move yarn instructions into each section to ensure it's not overlooked

docs/contributing/setup-linux.md

@@ -3,6 +3,7 @@
 You will need to install these tools on your machine:
 
  - Node.js v8.12.0 (also ensure you can build native modules)
+ - Yarn
  - Python 2.7
  - Electron dependencies
 
@@ -15,6 +16,20 @@ distributions and package managers.
 Find your distribution on [this list](https://nodejs.org/en/download/package-manager/)
 and follow the instructions to install the version you require.
 
+## Yarn
+
+Follow [this guide](https://yarnpkg.com/en/docs/install) to install
+a system-level `yarn` for your distribution. GitHub Desktop uses a local version
+of `yarn`, but it needs a version on your `PATH` to bootstrap itself.
+
+This is important because `yarn` uses lock files to pin dependencies. If you
+find yourself changing packages, this will prevent mismatches in versions
+between machines.
+
+If you're not familiar with `yarn`, please read [this document](./working-with-packages.md)
+to help familiarize yourself with how to do the common package tasks that are
+relevant to Desktop.
+
 ## Python 2
 
 Refer to your distributions package manager to obtain the latest version of the

docs/contributing/setup-macos.md

@@ -3,6 +3,7 @@
 You will need to install these tools on your machine:
 
  - Node.js v8.12.0
+ - Yarn
  - Python 2.7
  - Xcode and Xcode Command Line Tools (Xcode -> Preferences -> Downloads)
 
@@ -52,6 +53,20 @@ If you see `v8.12.0`, you're good to go.
 
 `asdf` is a little more involved to install. Check out the instructions [here](https://github.com/asdf-vm/asdf) and [here](https://github.com/asdf-vm/asdf-nodejs) for more information.
 
+## Yarn
+
+Follow [this guide](https://yarnpkg.com/en/docs/install#mac-stable) to install
+a system-level `yarn`. GitHub Desktop uses a local version of `yarn`, but it
+needs a version on your `PATH` to bootstrap itself.
+
+This is important because `yarn` uses lock files to pin dependencies. If you
+find yourself changing packages, this will prevent mismatches in versions
+between machines.
+
+If you're not familiar with `yarn`, please read [this document](./working-with-packages.md)
+to help familiarize yourself with how to do the common package tasks that are
+relevant to Desktop.
+
 ## Python
 
 macOS comes with Python pre-installed, and it happens to be the right version, so you're probably fine! But let's be sure. Open a terminal and run this command inside the Desktop source directory:

docs/contributing/setup-windows.md

@@ -3,6 +3,7 @@
 You will need to install these tools on your machine:
 
  - Node.js v8.12.0
+ - Yarn
  - Python 2.7
  - Visual C++ Build Tools
 
@@ -36,6 +37,20 @@ the top of your current install.
 
 _TODO: what options do we have here for Windows?_
 
+## Yarn
+
+Follow [this guide](https://yarnpkg.com/en/docs/install#windows-stable) to install
+a system-level `yarn`. GitHub Desktop uses a local version of `yarn`, but it
+needs a version on your `PATH` to bootstrap itself.
+
+This is important because `yarn` uses lock files to pin dependencies. If you
+find yourself changing packages, this will prevent mismatches in versions
+between machines.
+
+If you're not familiar with `yarn`, please read [this document](./working-with-packages.md)
+to help familiarize yourself with how to do the common package tasks that are
+relevant to Desktop.
+
 ## Python
 
 Open a shell and run this command:

docs/contributing/setup.md

@@ -12,21 +12,6 @@ Experimental support for building Desktop is also available for these platforms:
 
  - [ARM64](./building-arm64.md)
 
-## Install Yarn
-
-After doing this setup, you also need to install `yarn` as Desktop uses
-this for managing packages instead of NPM. **Do not install `yarn` through
-NPM**. Refer to the [install instructions](https://yarnpkg.com/en/docs/install)
-for your OS.
-
-This is important because `yarn` uses lock files to pin dependencies. If you
-find yourself changing packages, this will prevent mismatches in versions
-between machines.
-
-If you're not familiar with `yarn`, please read [this document](./working-with-packages.md)
-to help familiarize yourself with how to do the common package tasks that are
-relevant to Desktop.
-
 ## Verification
 
 Then verify you have these commands available in your shell and that the found