Mention prebuilt packages and expand build instructions

Fixes #202

manual/src/getting_started.md

@@ -1,10 +1,27 @@
 # Getting Started
 
-## Installation
+## Installing a prebuilt package
 
-The latest version of difftastic is [available on
-crates.io](https://crates.io/crates/difftastic). You can install it
-with Cargo. Difftastic requires Rust 1.56 or later.
+Prebuilt packages are available on the following platforms.
+
+[![Packaging status](https://repology.org/badge/vertical-allrepos/difftastic.svg)](https://repology.org/project/difftastic/versions)
+
+## Installing from source
+
+### Build Requirements
+
+Difftastic is written in Rust, so you will need Rust installed. I
+recommend [rustup](https://rustup.rs/) to install Rust. Difftastic
+requires Rust version 1.56 or later.
+
+You will also need a C++ compiler that supports C++14. I recommend
+Clang.
+
+### Build
+
+You can download and build [difftastic on
+crates.io](https://crates.io/crates/difftastic) with Cargo (which is
+part of Rust).
 
 ```
 $ cargo install difftastic