Change the docs license to CC BY-SA

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/42891

CONTRIBUTING.md

@@ -9,6 +9,10 @@ terms.
 
 [DCO + License](https://gitlab.com/gitlab-org/dco/blob/master/README.md)
 
+All Documentation content that resides under the [doc/ directory](/doc) of this
+repository is licensed under Creative Commons:
+[CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).
+
 _This notice should stay as the first item in the CONTRIBUTING.md file._
 
 ---

LICENSE

 The GitLab Enterprise Edition (EE) license (the “EE License”)
-Copyright (c) 2011-2017 GitLab B.V.
+Copyright (c) 2011-2018 GitLab B.V.
 
 With regard to the GitLab Software:
 
@@ -42,3 +42,8 @@ THE SOFTWARE.
 For all third party components incorporated into the GitLab Software, those
 components are licensed under the original license provided by the owner of the
 applicable component.
+
+---
+
+All Documentation content that resides under the doc/ directory of this
+repository is licensed under Creative Commons: CC BY-SA 4.0.