Add extended documentation on gateway compression

guides/advanced/gateway_compression.md

@@ -0,0 +1,50 @@
+# Gateway Compression
+
+Nostrum supports either the `zlib-stream` or `zstd-stream` gateway compression
+methods, as documented
+[here](https://discord.com/developers/docs/topics/gateway#encoding-and-compression)
+
+Most users are fine to leave the `gateway_compression` configuration option set
+to `:zlib` (default), but users looking for a potential reduction in payload
+sizes from the Discord gateway can optionally set `:zstd` here.
+
+## Using `:zstd` compression
+
+Using `:zstd` depends on the [`:ezstd`](https://hex.pm/packages/ezstd) library,
+so you will have to add this dependency to your `mix.exs` file:
+
+```elixir
+  defp deps do
+    [
+      {:nostrum, ...},
+      {:ezstd, "~> 1.1"} # new dependency
+    ]
+  end
+```
+
+
+> #### `:ezstd` NIFs {: .info}
+>
+> Some functionality of `:ezstd` depends on Erlang NIFs (Natively Implemented
+> Functions). This means that a proper compiler installation as well as other
+> build tools like `git` may be necessary at the stage where you compile your
+> dependencies.
+>
+> It may be useful to run `mix deps.compile` in any build systems to ensure that
+> your application does not need build utilities in the built application image.
+
+Once you have this additional dependency installed in your project, set the
+`:nostrum`, `:gateway_compression` configuration option to `:zstd` and Nostrum
+should pick up on it.
+
+You will need to run `mix deps.get` and `mix deps.compile` to install and
+compile the new `:ezstd` dependency.
+
+> #### Nostrum detection of `:ezstd` {: .tip}
+>
+> Since the check for `:ezstd` takes place when you compile Nostrum, you might
+> need to run `mix deps.compile --force nostrum` to ensure that Nostrum is
+> recompiled and recognises the newly installed `:ezstd` dependency.
+>
+> Not doing this may mean that your compiled Nostrum version is still using
+> dummy handlers that will error out even when `:ezstd` is installed.