adds GitHub PR and issue templates

.github/ISSUE_TEMPLATE.md

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+Thank you for taking your time to talk with us!
+
+**What is this issue about?**
+
+- [ ] Bug report
+- [ ] Feature request
+- [ ] Question
+
+
+**Description**
+
+
+<!-- In case of bug report, please fill out the following information. Is not mandatory, but will help up finding the cause of the issue quicker. -->
+**Environment information**
+
+* asciidoctor-maven-plugin version: ___
+* asciidoctorj version: ___
+<!-- Open a terminal and run `mvn -v`. Note your home path may appear, edit if necessary. -->
+* Maven, Java and OS version: ___
+
+
+

.github/PULL_REQUEST_TEMPLATE.md

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+Thank you for opening a pull request and contributing to asciidoctor-maven-plugin!
+<!-- Update "[ ]" to "[x]" to check a box -->
+
+**What kind of change does this PR introduce?** (check at least one)
+
+- [ ] Bugfix
+- [ ] Feature
+- [ ] Documentation
+- [ ] Refactor
+- [ ] Build improvement
+- [ ] Other (please describe)
+
+
+**What is the goal of this pull request?**
+
+
+**Are there any alternative ways to implement this?**
+
+
+**Are there any implications of this pull request? Anything a user must know?**
+
+
+**Is it related to an existing issue?**
+<!-- If so, please add a line of the form: `Fixes #Issue` --> 
+
+*Finally, please add a corresponding entry to CHANGELOG.adoc*