Add issue templates

Problem: by default, when creating an issue, user has to fill everything
from scratch and this is not good.

Solution: add issue templates for `task` and `bug`, refer people to
page for openning new issue in `CONTRIBUTING.md`.

.github/CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 ## Reporting issues
 
-Please [open an issue](../../../issues) if you find a bug or have a feature request.
+Please [open an issue](../../../issues/new/choose) if you find a bug or have a feature request.
 Note: you need to login (e. g. using your GitHub account) first.
 Before submitting a bug report or feature request, check to make sure it hasn't already been submitted
 

.github/ISSUE_TEMPLATE/bug.md

@@ -0,0 +1,41 @@
+---
+name: Bug report
+about: If you think our software behaves not the way it should, report a bug
+title: "[BUG]"
+labels: ''
+assignees: ''
+
+---
+
+# Description
+
+<!--
+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
+-->
+
+# Expected behavior
+
+<!--
+A clear and concise description of what you expected to happen.
+-->
+
+# Environment
+
+- OS
+- branch/revision
+
+# Additional context
+
+<!--
+Add any other context about the problem here.
+-->

.github/ISSUE_TEMPLATE/task.md

@@ -0,0 +1,20 @@
+---
+name: Task
+about: Suggest a task for this project
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+# Clarification and motivation
+
+<!--
+Clarify what you want to be done and why.
+-->
+
+# Acceptance criteria
+
+<!--
+Clarify how we can verify that the task is done.
+-->