Add macOS and Windows-specific getting started pages

I always forget how to upgrade Git on macOS, and Stack Overflow is down so ... add it to my book..

src/SUMMARY.md

@@ -3,6 +3,8 @@
 * [Introduction](introduction.md)
 * [Notes and warnings](notes.md)
 * [Getting Started](getting-started.md)
+	* [macOS](getting-started/macos.md)
+	* [Windws](getting-started/windows.md)
 * [Configuration](configuration.md)
 	* [User information](configuration/user.md)
 	* [Git configuration](configuration/git.md)

src/getting-started/macos.md

@@ -0,0 +1,19 @@
+# Getting started on macOS
+
+Use [Homebrew](https://brew.sh/).
+
+## Install Git
+```bash
+brew install git
+```
+
+## Update Git
+If you haven't updated brew in a while, you can both make sure it and all applications are current.
+```bash
+brew update && brew upgrade
+```
+
+Otherwise just upgrade Git itself:
+```bash
+brew upgrade git
+```

src/getting-started/windows.md

@@ -0,0 +1,5 @@
+# Getting started on Windows
+There's a lot of different options. In no particular order:
+
+* [Git for Windows](https://git-scm.com/download/win)
+* [Cmder](https://cmder.net/)