Improve troubleshooting doc/flows

- Use better wording for unknown server error
- Add basic troubleshooting doc

docs/troubleshooting.md

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+# Troubleshooting
+- [Purpose](#purpose)
+- [Logs](#logs)
+  - [Locations](#locations)
+  - [Reading Them](#reading-them)
+  - [Common issues](#common-issues)
+- [Submit an issue](#submit-an-issue)
+
+## Purpose
+This document describes basic troubleshooting steps for mxisd.
+
+## Logs
+### Locations
+mxisd logs to `STDOUT` (Standard Output) and `STDERR` (Standard Error) only, which gets redirected
+to log file(s) depending on your system.
+
+If you use the [Debian package](install/debian.md), this goes to `syslog`.  
+If you use the [Docker image](install/docker.md), this goes to the container logs.  
+
+For any other platform, please refer to your package maintainer.
+
+### Reading them
+Before reporting an issue, it is important to produce clean and complete logs so they can be understood.
+
+It is usually useless to try to troubleshoot an issue based on a single log line. Any action or API request
+in mxisd would trigger more than one log lines, and those would be considered necessary context to
+understand what happened.
+
+You may also find things called *stacktraces*. Those are important to pin-point bugs and the likes and should
+always be included in any report. They also tend to be very specific about the issue at hand.
+
+Example of a stacktrace:
+```
+Exception in thread "main" java.lang.NullPointerException
+        at com.example.myproject.Book.getTitle(Book.java:16)
+        at com.example.myproject.Author.getBookTitles(Author.java:25)
+        at com.example.myproject.Bootstrap.main(Bootstrap.java:14)
+```
+
+### Common issues
+#### Internal Server Error
+`Contact your administrator with reference Transaction #123456789`
+
+This is a generic message produced in case of an unknown error. The transaction reference allows to easily find
+the location in the logs to look for an error.
+
+**IMPORTANT:** That line alone does not tell you anything about the error. You'll need the log lines before and after,
+usually including a stacktrace, to know what happened. Please take the time to read the surround output to get
+context about the issue at hand.
+
+## Submit an issue
+In case the logs do not allow you to understand the issue at hand, please submit clean and complete logs
+as explained [here](#reading-them) in a new issue on the repository, or [get in touch](../README.md#contact).