Add merge request template, closes #3

.gitlab/merge_request_templates/merge_into_master.md

+<!--
+Please fill out this template fully! Remember that the master branch must 
+always be stable. So take great care in documenting and testing your changes. 
+
+For info on how this package uses versioning see https://r-pkgs.org/description.html#version
+-->
+
+### description
+<!--
+Describe or list the changes here.
+Comment on any notes that come up during devtools::check().
+-->
+
+### system details
+* RStudio Version:
+* R Version:
+* OS Version:
+
+### checklist
+<!-- Please check `[x]` the applicable boxes. -->
+* [ ] This merge includes bug fixes or changes in the functionality of the package (whether they are user-visible or not).
+    * [ ]  update documentation (if necessary)
+    * [ ]  update NEWS.md
+    * [ ]  update version in DESCRIPTION
+    * [ ]  update date in DESCRIPTION
+* [ ]  This merge introduces new features.
+    * [ ]  new features are documented
+    * [ ]  new features have tests
+* [ ]  `devtools::document()`
+* [ ]  `devtools::check()`: no errors, no warnings
+
+<!--
+If this merge introduces a new version (major, minor or patch) to the 
+master branch do these steps after the branch has been successfully merged:
+- git tag
+- build binary and source package
+- gitlab release including the builds
+-->
\ No newline at end of file