Document installation on Windows 7

CONTRIBUTING.md

@@ -29,24 +29,31 @@ still useful, but it doesn't support `.nvmrc` files.
 
 Then you need `git`, if you don't have that yet: https://git-scm.com/
 
-And for the final step before we actually get started, you'll need build tools to install
-the native modules used by the application. On Windows, it's easiest to open the [Command
-Prompt (`cmd.exe`) as Administrator](<https://technet.microsoft.com/en-us/library/cc947813(v=ws.10).aspx>)
-and run this:
+### macOS
 
-```
-npm install --global --production windows-build-tools
-```
+1.  Install the [Xcode Command-Line Tools](http://osxdaily.com/2014/02/12/install-command-line-tools-mac-os-x/).
 
-On OSX you can install the [XCode Command-line tools](http://osxdaily.com/2014/02/12/install-command-line-tools-mac-os-x/). On Linux you'll need to take a trip to your
-favorite package manager. Python 2.x and GCC are two key necessary components.
+### Windows
+
+1.  **Windows 7 only:** Install Microsoft .NET Framework 4.5.1:
+    https://www.microsoft.com/en-us/download/details.aspx?id=40773
+1.  Install _Windows Build Tools_: Open the [Command Prompt (`cmd.exe`) as Administrator](<https://technet.microsoft.com/en-us/library/cc947813(v=ws.10).aspx>)
+    and run: `npm install --global --production --add-python-to-path windows-build-tools`
+
+### Linux
+
+1.  Pick your favorite package manager.
+1.  Install Python 2.x.
+1.  Install GCC.
+
+### All platforms
 
 Now, run these commands in your preferred terminal in a good directory for development:
 
 ```
 git clone https://github.com/signalapp/Signal-Desktop.git
 cd Signal-Desktop
-npm install --global yarn      # (only if you don’t already have yarn)
+npm install --global yarn      # (only if you don’t already have `yarn`)
 yarn install --frozen-lockfile # Install and build dependencies (this will take a while)
 yarn grunt                     # Generate final JS and CSS assets
 yarn icon-gen                  # Generate full set of icons for Electron