[README] Expand the Build Requirements section.

Two common issues of those on the gitter.im channel when trying to
build the xamarin-android repo are:

 1. Not using Mono 4.4.
 2. xxd(1) isn't installed.

Update the Build Requirements section to better detail the other
software packages which need to be installed in order to build the
xamarin-android repo.

Author: jonpryor

Diff for: README.md

@@ -40,14 +40,54 @@ Overridable MSBuild properties include:
 
 # Build Requirements
 
-Building Xamarin.Android requires the Java Development Kit (JDK), several
-pieces of the Android SDK, and the Android NDK.
+Building Xamarin.Android requires:
+
+* [Mono 4.4 or later](#mono-sdk)
+* [The Java Development Kit (JDK)](#jdk)
+* [Autotools (`autoconf`, `automake`, etc.)](#autotools)
+* [`xxd`](#xxd)
+* [The Android SDK and NDK](#ndk)
+
+<a name="mono-sdk" />
+## Mono MDK
+
+Mono 4.4 or later is required to build on [OS X][osx-mono] and Linux.
+
+(This is because the build system uses the [XmlPeek][xmlpeek] task, which
+was first added in Mono 4.4.)
+
+[osx-mono]: http://www.mono-project.com/download/#download-mac
+[xmlpeek]: https://msdn.microsoft.com/en-us/library/ff598684.aspx
+
+<a name="jdk" />
+## Java Development Kit
 
 The Java Development Kit may be downloaded from the
 [Oracle Java SE Downloads page][download-jdk].
 
 [download-jdk]: http://www.oracle.com/technetwork/java/javase/downloads/
 
+<a name="autotools" />
+## Autotools
+
+Autotools -- including `autoconf` and `automake` -- are required to build
+the Mono runtimes.
+
+On OS X, autotools are distributed with [Mono.framework][osx-mono].
+
+<a name="xxd" />
+## `xxd`
+
+The [xxd][xxd] utility is used to build [src/monodroid](src/monodroid).
+It is installed by default on OS X. Linux users may need to separately
+install it; it may be part of the [vim-common` package][sid-vim-common].
+
+[xxd]: http://linux.die.net/man/1/xxd
+[sid-vim-common]: https://packages.debian.org/sid/vim-common
+
+<a name="ndk" />
+## Android NDK, SDK
+
 To simplify building Xamarin.Android, important pieces of the Android SDK
 and Android NDK will be automatically downloaded and installed from
 Google's website. Downloaded files are cached locally, by default into