Create ISSUE_TEMPLATE.md

Add bug template to be used for reporting issues.

ISSUE_TEMPLATE.md

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+Thank you for opening an issue with Mailu. Please understand that issues are meant for bugs and enhancement-requests.  
+For **user-support questions**, reach out to us  on [matrix](https://matrix.to/#/#mailu:tedomum.net).
+
+To be able to help you best, we need some more information.
+
+## Before you open your issue
+- [ ] Check if no issue or pull-request for this already exists.
+- [ ] Check [documentation](https://mailu.io/master/) and [FAQ](https://mailu.io/master/faq.html). (Tip, use the search function on the documentation page)
+- [ ] You understand `Mailu` is made by volunteers in their **free time** — be conscise, civil and accept that delays can occur.
+- [ ] The title of the issue should be short and simple. It should contain specific terms related to the actual issue. Be specific while writing the title.
+
+## Environment & Versions
+Environment:
+ - [ ] docker-compose
+ - [ ] kubernetes
+ - [ ] docker swarm
+
+Versions:
+To find your version, get the image name of a mailu container and read  the version from the tag (example for version 1.7).  
+`docker ps -a | grep mailu`  
+`140b09d4b09c    mailu/roundcube:1.7    "docker-php-entrypoi…"    2 weeks ago    Up 2 days (healthy)    80/tcp`
+
+## Description
+<Further explain the bug in a few words. It should be clear what the unexpected behaviour is.  Share it in an easy-to-understand language.>
+
+## Replication Steps
+< Steps for replicating your issue>
+
+## Expected behaviour
+<Explain what results you expected - be as specific as possible. Just saying "it doesn’t work as expected" is not useful. It's also helpful to describe what you actually experienced. >
+																																																																	
+	
+## Logs
+Often it is very useful to include log fragments of the involded component. You can get the logs via `docker logs <container name> --tail 1000`. For example for the admin container:  
+`docker logs mailu_admin_1 --tail 1000`  
+or using docker-compose `docker-compose -f /mailu/docker-compose.yml logs --tail 1000 admin`