Adding coversheet contribution description template

.gitlab/merge_request_templates/coversheet_contribution.md

+| Coversheet | Value |
+| ------ | ------ |
+| Meeting ID | #MEETING_ID# |
+| Source | #SOURCE# |
+| CR Title | #CR_TITLE# |
+| CR Against Release | #RELEASE# |
+| CR Against TS/TR | #TS_AGAINST# |
+| Work-item | #WI# |
+| Reason | #REASON# |
+| Type of change | Editorial |
+| Clauses | #CLAUSES# |
+| Other comments | |