Contributing: Choose an appropriate logging level

Open to comments on my definitions.

Contributing.md

@@ -312,6 +312,43 @@ exception*.  e.g. Logging will know you were in your `get_foo()` function
 but you should still tell it what actually (failed to have) happened
 in there.
 
+### Use the appropriate logging level
+You must ensure necessary information is always in the log files, but 
+not so much that it becomes more difficult to discern important information 
+when diagnosing an issue.
+
+`logging`, and thus our `logger` instances provide functions of the 
+following names:
+
+- `info` - For general messages that don't occur too often outside of startup 
+  and shutdown.
+- `warning` - An error has been detected, but it doesn't impact continuing 
+  functionality.  In particular **use this when logging errors from 
+  external services**.  This would include where we detected a known issue 
+  with Frontier-supplied data.  A currently unknown issue *may* end up 
+  triggering logging at `error` level or above.
+- `error` - An error **in our code** has occurred.  The application might be 
+  able to continue, but we want to make it obvious there's a bug that we 
+  need to fix.
+- `critical` - An error has occurred **in our code** that impacts the 
+  continuation of the current process.
+- `debug` - Information about code flow and data that is occurs too often
+  to be at `info` level.  Keep in mind our *default* logging level is DEBUG,
+  but users can change it for the
+  [plain log file](https://github.com/EDCD/EDMarketConnector/wiki/Troubleshooting#plain-log-file),
+  but the
+  [debug log giles](https://github.com/EDCD/EDMarketConnector/wiki/Troubleshooting#debug-log-files)
+  are always at least at DEBUG level.
+  
+In addition to that we utilise one of the user-defined levels as:
+
+- `trace` - This is a custom log level intended for debug messages which 
+  occur even more often and would cause too much log output for even 
+  'normal' debug use. 
+  In general only developers will set this log level, but we do supply a
+  command-line argument and `.bat` file for users to enable it.  It cannot be
+  selected from Settings in the UI.
+
 ## Prefer fstrings to modulo-formatting and .format
 
 [fstrings](https://www.python.org/dev/peps/pep-0498/) are new in python 3.6, and allow for string interpolation rather