Add the first FAQ on how to use git sparse checkout

README

-test  5 for hysop commits ...
+
+FAQ:
+
+1. How to get only the code from the git repository?
+Create a local directory to clone into
+  mkdir my_hysop && cd  my_hysop
+Initialise an empty git local repository
+  git init
+Configure it for sparse checkout
+  git config core.sparsecheckout true
+Add the remote information
+  git remote add -f origin git+ssh://<USERNAME>@scm.forge.imag.fr/var/lib/gforge/chroot/scmrepos/git/hysop/hysop.git
+Select the HySoP directory (that contains all the code)
+  echo HySoP/ >> .git/info/sparse-checkout
+Get all files (only from the HySoP directory)
+  git checkout master
+
+Remarks:
+Solution comes from http://stackoverflow.com/questions/4114887/is-it-possible-to-do-a-sparse-checkout-without-checking-out-the-whole-repository
+It keeps the directory/files tree as git sees all files as a single unit.
+(see http://stackoverflow.com/questions/2336580/sparse-checkout-in-git-1-7-0)