Add documentation for per-channel module/command disabling

Upcoming feature in Sopel 7. See sopel-irc/sopel#1235

_usage/per-channel-configuration.md

@@ -0,0 +1,55 @@
+---
+title: Per-channel configuration
+order: 50
+---
+
+**Note: This guide applies to Sopel 7, which is still in development. You can
+try out these features by [running Sopel from source]({{ site.repo
+}}#latest-source).**
+
+If your Sopel bot is in many channels, you've probably run into at least one
+case where a module that one channel loves is hated by the users and/or
+operators in another. This is pretty common—after all, every IRC channel is a
+fiefdom with its own dictator, who may or may not be benevolent.
+
+Fortunately, configuring Sopel to work around such situations is easy.
+
+## Channel config sections
+
+In Sopel's config file, you can add sections for channels you want to treat
+specially. For example:
+
+```cfg
+[#FunHaters]
+disable_modules = emoticons,instagram,reddit,xkcd
+disable_commands = {"dice": ["roll", "choose"], "url": ["title_auto"]}
+```
+
+The channel name is taken literally; no pattern-matching is supported, for now.
+
+In channels named `#FunHaters`, this would disable the `emoticons`, `instagram`,
+`reddit`, and `xkcd` modules entirely; it would also prevent the automatic
+title-fetcher from `url` from running (but still allow the manual `.title`
+command), and block use of the `.roll` and `.ch` commands from the `dice`
+module. Details for each option below.
+
+### `disable_modules`
+
+The `disable_modules` option is a simple comma-separated list of module names,
+just like [`core.enable`]({{ site.docs
+}}config.html#sopel.config.core_section.CoreSection.enable) or
+[`core.exclude`]({{ site.docs
+}}config.html#sopel.config.core_section.CoreSection.exclude).
+
+### `disable_commands`
+
+The `disable_commands` option is written as a Python dict, where each key is a
+module name and each value is a list of method names within that module.
+
+Be sure to use the _method_ name, not the _command_ name. Matching on method
+names makes it possible to disable things like URL handlers, too.
+
+*Note: A future version of Sopel might move to a YAML config file format instead
+of the INI format it uses now, to simplify the inclusion of more complex,
+structured options like this. Let us know [on
+IRC](irc://irc.freenode.net/#sopel) if you think this is a good idea.*