🔧 update issue templates

.github/ISSUE_TEMPLATE/Bug_report.md

+---
+name: Bug report
+about: Create a report to help us improve
+
+---
+
+- What is the issue you have?
+
+- Please describe the steps to reproduce the issue. Can you provide a small but working code example?
+
+- What is the expected behavior?
+
+- And what is the actual behavior instead?
+
+- Which compiler and operating system are you using? Is it a [supported compiler](https://github.com/nlohmann/json#supported-compilers)?
+
+- Did you use a released version of the library or the version from the `develop` branch?
+
+- If you experience a compilation error: can you [compile and run the unit tests](https://github.com/nlohmann/json#execute-unit-tests)?

.github/ISSUE_TEMPLATE/Feature_request.md

+---
+name: Feature request
+about: Suggest an idea for this project
+
+---
+
+- Describe the feature in as much detail as possible.
+
+- Include sample usage where appropriate.