Add a link to the build guide to README.md

README.md

@@ -53,10 +53,14 @@ We operate an IRC channel and a Discord server for discussing 86Box, its develop
 [![Visit our Discord server](https://discordapp.com/api/guilds/262614059009048590/embed.png)](https://discord.gg/QXK9XTv)
 
 Contributions
----------
+-------------
 
 We welcome all contributions to the project, as long as the [contribution guidelines](CONTRIBUTING.md) are followed.
 
+Building
+---------
+For instructions on how to build 86Box from source, see the [build guide](https://86box.readthedocs.io/en/latest/dev/buildguide.html).
+
 Licensing
 ---------