doc: enhance GitHub template.

- add issue and PR guide.
- rewrite templates

.github/how-to-write-a-good-issue.md

@@ -0,0 +1,25 @@
+# How to write a good issue
+
+Please keep in mind that the GitHub issue tracker is not intended as a general support forum, but for reporting bugs and feature requests.
+
+For other type of questions, consider using one of:
+
+- the [Traefik community Slack channel](https://traefik.herokuapp.com)
+- [StackOverflow](https://stackoverflow.com/questions/tagged/traefik)
+
+## Title
+
+The title must be short and descriptive.
+
+## Description of the issue
+
+- Explain the conditions which led you to write this issue: the context.
+- The context should lead to something, an idea or a problem that you’re facing.
+- Remain clear and concise.
+
+## Format your messages
+
+Help the reader focus on what matters and understand the structure of your message.
+[GitHub flavored markdown](https://help.github.com/articles/github-flavored-markdown) has a simple but effective syntax: make use of bold text, lists, check-boxes, syntax highlighting, headers, lists, links and images.
+
+