Update issue templates

.github/ISSUE_TEMPLATE/issue-template.md

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+---
+name: Issue Template
+about: This template provides guidelines on how to structure an issue for the controlled-vocabularies
+  repository
+title: "[CONTROLLED_VOCABULARY_NAME] My Issue Title"
+labels: ''
+assignees: marioscrock
+
+---
+
+When adding a new issue, please specify the name of the controlled vocabulary you refer to by replacing the right value in the issue title between square brackets. 
+Replace  `CONTROLLED_VOCABULARY_NAME` with one of the following values:  `application-layer-protocol`, `communication-method`, `conditions-for-access-and-usage`, `mobility-theme`, `mobility-data-standard`, `georeferencing-method`, `grammar`, `network-coverage`, ` intended-information-service`, `transport-mode`, `update-frequency`. If more than one values are applicable insert them in the title using an additional `[CONTROLLED_VOCABULARY_NAME]`. 
+
+If the issue is generic and valid for all the controlled vocabularies, you can remove the part between square brackets.
+
+Provide a short title summarising the issue, and a full description of the use case and/or the proposal to be implemented.