Add guide for building the docs

docs/README.md

@@ -0,0 +1,16 @@
+# Instructions for Building Documentation
+
+The documentation for the Discord.NET library uses [docfx][docfx-main]. [Instructions for installing this tool can be found here.][docfx-installing]
+
+1. Navigate to the root of the repository.
+2. (Optional) If you intend to target a specific version, ensure that you
+have the correct version checked out.
+3. Build the library. Run `dotnet build` in the root of this repository.
+ Ensure that the build passes without errors.
+4. Build the docs using `docfx .\docs\docfx.json`. Add the `--serve` parameter
+to preview the site locally. Some elements of the page may appear incorrect
+when not hosted by a server.
+  - Remarks: According to the docfx website, this tool does work on Linux under mono.
+
+[docfx-main][https://dotnet.github.io/docfx/]
+[docfx-installing][https://dotnet.github.io/docfx/tutorial/docfx_getting_started.html]