docs: start doc page on the build system

docs/dev/build.rst

+Build system
+============
+
+This page explains internals of the Gluon build system. It is currently very
+incomplete; please contribute if you can!
+
+Feed management
+---------------
+
+Rather that relying on the *feed.conf* mechanism of OpenWrt directly, Gluon
+manages its feeds (*"modules"*) using a collection of scripts. This solution was
+selected for multiple reasons:
+
+- Feeds lists from Gluon base and the site repository are combined
+- Patchsets are applied to downloaded feed repositories automatically
+
+The following variables specifically affect the feed management:
+
+GLUON_FEEDS
+    List of base feeds; defined in file *modules* in Gluon base
+
+GLUON_SITE_FEED
+    List of site feeds; defined in file *modules* in site config
+
+\*_REPO, \*_BRANCH, \*_COMMIT
+    Git repository URL, branch and and
+    commit ID of the feeds to use. The branch name may be omitted; the default
+    branch will be used in this case.
+
+GLUON_BASE_FEEDS
+    Additional feed definitions to be added to *feeds.conf*
+    verbatim. By default, this contains a reference to the Gluon base packages;
+    when using the Gluon build system to build a non-Gluon system, the variable
+    can be set to the empty string.

docs/index.rst

@@ -42,6 +42,7 @@ Several Freifunk communities in Germany use Gluon as the foundation of their Fre
    dev/wan
    dev/mac_addresses
    dev/site_library
+   dev/build
 
 .. toctree::
    :caption: gluon-web Reference