Add clarifications and instructions to README.

README.md

@@ -12,6 +12,41 @@ documentation](https://developers.google.com/poly/develop/unity).
 
 _Unity is a trademark of Unity Technologies._
 
+## Installing Poly Toolkit
+
+If you are a _user_ of Poly Toolkit (most common case), you probably do not
+need to download and build the source code. You can simply download a
+[pre-built package](https://github.com/googlevr/poly-toolkit-unity/releases).
+
+More instructions are available in the [online
+documentation](https://developers.google.com/poly/develop/unity).
+
+Note: If you are using Unity 2018 or above, you will need to enable the
+_unsafe code_ option in **Player Settings** before installing the package.
+This is because Poly Toolkit uses `unsafe {}` code blocks for direct
+pointer manipulation when parsing files, for performance reasons.
+
+## Building from Source
+
+If the pre-built packages do not suit your needs (for example, you want the
+cutting edge features that are checked in but not yet part of the official
+releases), you can build from source. To do that:
+
+* Install Unity 5.6.3 or later. Note: if you are not using the exact version of
+  Unity that Poly Toolkit was developed with, you will get a warning to
+  not check in any code, but you can ignore that, because you are just
+  building it, not modifying the code.
+
+* Clone this repo to your machine.
+
+* Open it in Unity.
+
+* Click **Poly > Dev > Build .unitypackage** on the menu.
+
+* This will create a **unitypackage** file.
+
+Then, simply use this package file to install Poly Toolkit in any project.
+
 ## License
 
 For license information, see the `LICENSE` file.