introduce issue and pull request templates (#3493)

* introduce issue and pull request templates
Signed-off-by: zouyee <zounengren@cmss.chinamobile.com>

* introduce issue and pull request templates
Signed-off-by: zouyee <zounengren@cmss.chinamobile.com>

.github/ISSUE_TEMPLATE/bug-report.md

+---
+name: Bug Report
+about: Report a bug encountered while using CoreDNS 
+labels: bug
+
+---
+
+<!-- Please use this template while reporting a bug and provide as much info as possible. Not doing so may result in your bug not being addressed in a timely manner. Thanks!
+
+If the matter is security related, please disclose it privately via security@coredns.io
+-->
+
+**What happened**:
+
+**What you expected to happen**:
+
+**How to reproduce it (as minimally and precisely as possible)**:
+
+**Anything else we need to know?**:
+
+**Environment**:
+
+- the version of CoreDNS:
+- Corefile:
+- logs, if applicable:
+- OS (e.g: `cat /etc/os-release`):
+- Others:

.github/ISSUE_TEMPLATE/enhancement.md

+---
+name: Enhancement Request
+about: Suggest an enhancement to the CoreDNS project
+labels: enhancement
+
+---
+<!-- Please only use this template for submitting enhancement requests -->
+
+**What would you like to be added**:
+
+**Why is this needed**: