doc: Add missing step in the beginning of building from source doc

Change-Id: Id834d3de69c038f3cc1aee3c59c3607f42fd5b49
Signed-off-by: Anastasia Klimchuk <aklm@flashrom.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/86429
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

doc/dev_guide/building_from_source.rst

@@ -1,6 +1,9 @@
 Building from source
 ====================
 
+First thing, clone git repository for yourself (if you haven't done it already),
+see :ref:`git repo setup`.
+
 You're going to need the following tools to get started:
 
 * gcc or clang

doc/dev_guide/development_guide.rst

@@ -15,6 +15,8 @@ design.
 
 You can `look at the latest flashrom development efforts in Gerrit <https://review.coreboot.org/q/project:flashrom>`_.
 
+.. _git repo setup:
+
 Set up the git repository and dev environment
 =============================================