Add tip for cc7 & git2.9

@airibarr , here is a small note about the git issues we discussed some time ago on CC7.

README.md

@@ -241,6 +241,26 @@ build_rpm7:
 
 Your spec file should also contain a `BuildRequires: golang`.
 
+Finally, the `git` version available on CC7 can produce errors like the following while fetching Go dependencies.
+
+```
+fatal: git fetch-pack: expected shallow list
+```
+
+One solution to this issue is to use `rh-git29` software collection. To do that, you should define a `before_script` similar to the following one:
+
+```
+  before_script:
+    (...)
+    - yum-config-manager --add-repo "http://linuxsoft.cern.ch/cern/centos/7/sclo/x86_64/rh"
+    - yum install -y --nogpgcheck rh-git29
+    - export PATH=/opt/rh/rh-git29/root/usr/bin:$PATH
+    - export LD_LIBRARY_PATH=/opt/rh/httpd24/root/usr/lib64:$LD_LIBRARY_PATH
+    - export MANPATH=/opt/rh/rh-git29/root/usr/share/man:$MANPATH
+    - export PERL5LIB=/opt/rh/rh-git29/root/usr/share/perl5/vendor_perl
+```
+The previous could be simplified by calling `source /bin/scl_source enable rh-git29`, but unfortunately it seems to fail.
+
 ### Building Maven packages
 
 Normal Maven builds will not work in Koji because Koji builders don't