Update issue templates

Provide Scyther-specific bug report template.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,38 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. Go to '...'
+2. Click on '....'
+3. Scroll down to '....'
+4. See error
+
+Please attach any files (with comments) needed to reproduce.
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**Scyther version used:**
+ - Version number, Scyther type [normal/compromising Adversaries/...]
+ - Github commit id (if available)
+
+**Platform information (please complete the following information):**
+ - OS: [e.g. iOS/MAC, Windows, Linux]
+ - Version [e.g. Windows 10, Ubuntu 20.04, MacOS version]
+ - If applicable, version numbers of graphviz, python, wxpython.
+
+**Additional context**
+Add any other context about the problem here.