Pool size config documentation (#397)

README.md

@@ -202,6 +202,12 @@ Req.get!(req, url: "https://api.github.com/user").body["login"]
 
 See [`Req.Request`] module documentation for more information on low-level API, request struct, and developing plugins.
 
+## Configuration
+
+Req supports many configuration options, see [`Req.new/1`] for a full list and see each step for
+more details. In particular, if you are looking for slightly lower level HTTP options such as
+timeouts, pool sizes, and certificates, see the [`run_finch`] documentation.
+
 ## Related Packages
 
 There are many packages that extend the Req library. To get yours listed here, send a PR.

lib/req/steps.ex

@@ -714,6 +714,24 @@ defmodule Req.Steps do
 
       Req.get!("https://httpbin.org/json", finch: MyFinch)
 
+  More commonly you'd add the the custom Finch pool as part of your supervision tree in your
+  `application.ex`:
+
+      children = [
+        {Finch,
+         name: MyFinch,
+         pools: %{
+           default: [size: 70]
+         }}
+      ]
+
+  That way you can also configure a bigger pool size for the HTTP pool. You just mustn't forget to
+  pass along `finch: MyFinch` as discussed above. You could use `Req.default_options/1` to make it
+  a global default but it's generally discouraged.
+
+  For documentation about the possible pool options and their meaning, please check out the
+  [Finch docs on pool configuration options](https://hexdocs.pm/finch/Finch.html#start_link/1-pool-configuration-options).
+
   ## Request Options
 
     * `:finch` - the name of the Finch pool. Defaults to a pool automatically started by Req.