meta: add template for bug reports (#1637)

.github/ISSUE_TEMPLATE/bug_report.md

+---
+name: Bug report
+about: Report a bug in a recent version of Gluon
+label: bug
+---
+
+<!--
+
+Please carefully fill out the questionaire below to help improve the
+timely triaging of issues. Walk through the questions below and use
+them as an inspiration for what information you can provide.
+
+Make use of codeblocks (three backticks before and after) where
+appropriate (configuration excerpts, log output, etc.). Example:
+
+```
+your code goes here
+```
+
+You can use the "Preview" tab to check how your issue is going to look
+before you actually send it in.
+
+Thank you for taking the time to report a bug with the Gluon project.
+
+-->
+
+### Bug report
+
+**What is the problem?**
+<!-- 
+- What is not working as expected?
+- How is it misbehaving?
+- When did the problem first start showing up?
+- What were you doing when you first noticed the problem?
+- On which devices (vendor, model and revision) is it misbehaving?
+- Does the issue appear on multiple devices or targets?
+-->
+
+**What is the expected behaviour?**
+<!--
+- How do you think it should work instead?
+- Did it work like that before?
+-->
+
+**Gluon Version:**
+<!-- 
+Please provide a usable Git reference before applying custom patches:
+
+By using a Git reference:
+    $ git describe --always
+    v2018.2-17-g3abadc28
+
+Or the URL to the relevant Gluon commit
+    https://github.com/freifunk-gluon/gluon/commit/<commit hash here>
+-->
+
+**Site Configuration:**
+<!--
+Please provide the URL to your site configuration repository and the
+explicit commit used to build the firmware experiencing the problem.
+ 
+Additionally excerpts of problem-related configuration parts are 
+often helpful. 
+-->
+
+**Custom patches:**
+<!--
+Be upfront about any custom patches you have applied to your Gluon build, as they might
+be part of your problem.
+-->
+