Add KojiResolver doc.

docs/KOJI_RESOLVER.rst

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+Koji Resolver
+=============
+
+MBS supports multiple methods of determining which modules are available to satisfy the
+buildrequires of a submitted module build. Each method is implemented as a derived class
+of ``GenericResolver`` in MBS, and can be configured using the ``resolver`` option in the MBS
+configuration file.
+
+This document describes ``KojiResolver`` and how it influences module builds in MBS.
+
+
+Enabling Koji Resolver
+======================
+
+Koji Resolver is enabled in the MBS configuration file using the ``RESOLVER = "koji"`` option,
+but this configuration is not enough to enable it for submitted module builds. It also needs
+to be enabled in the modulemd definition of the buildrequired base module of the submitted module
+build.
+
+This is done by adding the ``koji_tag_with_modules`` option to the ``xmd`` section of a base module
+definition. For example:
+
+.. code-block:: yaml
+
+    document: modulemd
+    version: 1
+    data:
+        xmd:
+            mbs:
+                koji_tag_with_modules: rhel-8.1.0-modules-build
+
+This option defines the Koji tag from where the buildrequires for the submitted module
+build will be taken. All module builds submitted against this base module stream will then use
+Koji Resolver.
+
+In case this option is not defined and Koji Resolver is enabled in the MBS configuration
+file, MBS simply falls back to using the default resolver (``DBResolver``).
+
+
+Koji Tag With Modules
+=====================
+
+The ``koji_tag_with_modules`` option mentioned above, defines the Koji tag with the modules
+available as buildrequires for a submitted module build that buildrequires this base module stream.
+
+This Koji tag inheritance should reflect the compatibility between different base modules.
+For example, if there is a ``platform:f31-server`` base module which should further extend
+the ``platform:f31`` module, the Koji tag of ``platform:f31-server`` should inherit the Koji tag of
+``platform:f31``.
+
+That way, all the modules built against ``platform:f31`` are available in the buildroot
+for modules built against ``platform:f31-server``.
+
+
+MBS Module Builds With KojiResolver
+===================================
+
+When KojiResolver is used for a particular base module, MBS will change the way it determines the
+available modules to be used as buildrequires as follows:
+
+- It does not try to find compatible base modules using virtual streams. The compatible
+  base modules are defined by using Koji tag inheritance.
+- It only uses module builds tagged in the tag defined in ``koji_tag_with_modules`` as possible
+  buildrequires, and therefore, also as input to Module Stream Expansion.
+- It reuses already built components only from the modules tagged in the tag defined in
+  ``koji_tag_with_modules``.