#1410 Add a readme and rename the script.

ExternalTools/Gitlab-CI/README.md

+- common.yml encompasses the stuff that is used in all Yaml files (template to provide compilation and check warnings jobs for macos and linux, the jobs to handle Doxygen documentation).
+- generate_yaml.py is a script to generate a tailored Yaml file.
+
+A possible use is for instance to type:
+
+python generate_configuration.py > my-gitlab-ci.yml
+
+which will put the configuration displayed in the main of this program in the target Yaml file.
+
+- gitlab-ci-no-macos.yml and gitlab-ci.yml are two such outputs; both provide 6 Linux cases and one of them add two macOS build on top of this.
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