Refine issue template for Github

Recently, there have been created a lot of issues which were not
actually bugs but simple questions.

It seems that people are too lazy to read the CONTRIBUTING file,
so improving the issue template with the important parts could
help to point the people to the right direction.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

ISSUE_TEMPLATE.md

@@ -1,13 +1,79 @@
-### libmodbus version
+Please read the following carefully before submitting this new issue.
 
-### Operating system
+ - Please ensure, that you are really reporting a bug. When in doubt,
+   post a message on https://groups.google.com/forum/#!forum/libmodbus
+   or send an email to libmodbus@googlegroups.com
 
-### Description of the Modbus network (server, client, links, etc)
+ - Please do not open issues to ask questions about using libmodbus.
+   Use the mailing list for this as there are many more people reading
+   that list, who could help you.
 
-### Expected behavior
+ - When using libmodbus from a distribution (Debian, Fedora...), please
+   report the bug first in the bug tracker of the distribution. The
+   reason for doing so is that the package maintainer should have a chance
+   to look at the issue first as it might be a packaging error. If/when
+   the package maintainer comes to the conclusion that is really an upstream
+   bug, then he/she will usually report it here by himself/herself.
+   This is because he/she is interested in staying in the notification chain
+   to decide about a backport as soon as a bugfix is available.
+   Otherwise you (distribution user) will be asked to do so explicitely.
 
-### Actual behavior
+When you get here and you are still convinced that you want report a bug:
 
-### Steps to reproduce the behavior (commands or source code)
+ - *Use a clear and decriptive title* for the issue to identify
 
-### libmodbus output with debug mode enabled
+ - *Which version of libmodbus are you using?* you can obtain this information
+   from your package manager or by running `pkg-config --modversion libmodbus`.
+   You can provide the sha1 of the commit if you have fetched the code with `git`.
+
+ - *Which operating system are you using?*
+
+ - *Describe the exact steps which reproduce the problem* in as many details as
+   possible. For example, the software/equipement which runs the Modbus server, how
+   the clients are connected (TCP, RTU, ASCII) and the source code you are using.
+
+ - *Enable the debug mode*, libmodbus provides a function to display the content
+   of the Modbus messages and it's very convenient to analyze issues
+   (http://libmodbus.org/docs/latest/modbus_set_debug.html).
+
+Good bug reports provide right and quick fixes!
+
+Finally, thank you very much for using libmodbus and taking the time to
+file a good bug report. Doing so signals your respect for the developers.
+
+The following template helps you to address the points above. Please delete
+everything up to and including the following line which starts with ---.
+
+---
+
+## libmodbus version
+
+  <libmodbus version or sha1 of the latest commit>
+
+## OS and/or distribution
+
+  <e.g. Windows, Linux... version>
+
+## Environment
+
+  <e.g. CPU architecture, 32 vs. 64 bit...>
+
+## Description
+
+  <...>
+
+## Expected behaviour
+
+  <...>
+
+## Actual behaviour
+
+  <...>
+
+## Steps to reproduce the behavior (commands or source code)
+
+  <...>
+
+## libmodbus output with debug mode enabled
+
+  <...>