Add manifest option documentation

Change-Id: I1b5a512d833715d59e47306b1632f4b84278249a

README.md

diff --git a/README.md b/README.md
index a93bad8..a0a869f 100644
--- a/README.md
+++ b/README.md
@@ -55,6 +55,8 @@
 - since, until, aggregate are the same defined in Gerrit Analytics plugin
     see: https://gerrit.googlesource.com/plugins/analytics/+/master/README.md
 - -u --url Gerrit server URL with the analytics plugins installed
+- -m --manifest Repo manifest XML path. Absolute path of the Repo manifest XML to import project
+from. Each project will be imported with the branch specified in the `revision` attribute.
 - -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. _Note: ElastiSearch 6.x
 requires this index format `name/type`, while from ElasticSearch 7.x just `name`_