Add documentation for the MockBuilder

doc/builders.md

@@ -1,5 +1,24 @@
 # Tito Builders
 
+## tito.builder.MockBuilder
+
+By default, Tito internally uses `rpmbuild` for building SRPM and RPM
+packages. This imposes some requirements on the host system - all the
+build dependencies need to be installed. Also, building RPM packages
+can potentially be dangerous and a badly written spec file can cause
+damage to the host system. If you prefer to use `mock` instead,
+configure `.tito/tito.props` like this:
+
+    [buildconfig]
+    builder = tito.builder.MockBuilder
+
+    [builder]
+    mock = fedora-rawhide-x86_64
+
+Alternatively, you can specify a mock chroot in the command line:
+
+    tito build --rpm --arg=mock=fedora-rawhide-x86_64
+
 ## tito.builder.FetchBuilder
 
 An unorthodox builder which can build packages for a git repo which does not actually have any tito footprint. The location of sources, and the version/release to assume we're building, come from a configurable strategy.

tito.props.5.asciidoc

@@ -154,6 +154,10 @@ tito.builder.Builder::
 Basic package builder. It create tar.gz of whole directory and create src.rpm
 and build rpm using some supported method.
 
+tito.builder.MockBuilder::
+Use `mock` instead of `rpmbuild` for building an RPM package.
+See doc/builders.md.
+
 tito.builder.NoTgzBuilder::
 Builder for packages that do not require the creation of tarball.
 Usually these package have source files checked directly into git.