Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "[docs] Add developer guidelines for contribution"
  - [docs] Add developer guidelines for contribution

    A new section was added to the devguide, stating
    expectations for code contributions.

    * What to include in a change
    * Code style guidelines
    * How to run tests

    JIRA: YARDSTICK-1335
    Change-Id: Idfe7790042a3a3fe6cd8763374b10f2268bd63cd
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index f64419c..a74d794 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit f64419cc7d16a679ed130dfe7592533b3cf9fdc1
+Subproject commit a74d79472dedf193e00905840d53cdee48e74f82