Update issue templates

In an attempt to enhance/professionalize our issue tracker, I've worked on some templates for issue submission. Please provide feedback.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,40 @@
+---
+name: Bug report
+about: Is something not working as it should? Please report it here.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!-- READ THIS FIRST
+Thank you for reporting an issue with RawTherapee. Before you continue, please check the following:
+1. Have you searched through the open and closed issues on GitHub for keywords related to your issue? If somebody else has already reported your issue, do not report it again. This is particularly important if you found an issue in a release version of RawTherapee, because many bugs will have been fixed already in the latest development builds.
+2. Have you read http://rawpedia.rawtherapee.com/How_to_write_useful_bug_reports ? This page describes how you can obtain as much useful information as possible to help the developers to quickly fix your issue.
+3. If your problem is related to a specific image file or edit, always (!) provide the image and corresponding sidecar (pp3) file.
+-->
+
+**Short description**
+<!-- Write a brief summary of the issue. Remove this section if you can fit the summary in the title of the issue. -->
+
+**Steps to reproduce**
+<!-- Provide the necessary steps to reproduce the issue. Using a list is not necessary, but recommended. For example:
+1. Go to '...'
+2. Click on '....'
+3. Scroll down to '....'
+4. See error -->
+
+**Expected behavior**
+<!-- Explain what you observe, what you expected and why you think this is not correct behavior. -->
+
+**Additional information**
+<!-- Some essential information to provide along with your description:
+- Version of RawTherapee
+- Version of your operating system
+- If you built from source, the contents of the `AboutThisBuild.txt` file in the installation folder
+
+More information to provide:
+- Screenshots or screencast
+- Image file + sidecar (pp3)
+- Stacktrace from a debug build
+-->