Documented extract branches option

'exctract-branches' option was missing from documentation.
It allows to extract and process branches information from the analytcis
plugin.

Change-Id: I5e1c3deb385643c2a4a9b88edb0c877717253f6e
diff --git a/README.md b/README.md
index 9f5dfa4..3bb8ce6 100644
--- a/README.md
+++ b/README.md
@@ -57,6 +57,7 @@
 - -u --url Gerrit server URL with the analytics plugins installed
 - -p --prefix (*optional*) Projects prefix. Limit the results to those projects that start with the specified prefix.
 - -e --elasticIndex Elastic Search index name. If not provided no ES export will be performed
+- -r --extract-branches Extract and process branches information (Optional) - Default: false
 - -o --out folder location for storing the output as JSON files
     if not provided data is saved to </tmp>/analytics-<NNNN> where </tmp> is
     the system temporary directory