Add guidelines for logging messages

HACKING.md

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+# Important notes on the code
+
+## Log messages
+
+### Messages
+
+- Keep your message in common format: `<problem>: <problematic value/description>`
+- If you have to write text, single quote values in your sentence.
+
+### Levels
+
+For logging, there are printf-like macros `LOG_(E|C|W|M|I|D)`.
+
+- `LOG_E` (ERROR):
+    - All messages, which lead to immediate abort and are caused by a programming error. The program needs patching and the error is not user recoverable.
+    - e.g.: Switching over an enum, `LOG_E` would go into the default case.
+- `LOG_C` (CRITICAL):
+    - The program cannot continue to work. It is used in the wrong manner or some outer conditions are not met.
+    - e.g.: `-config` parameter value is unreadable file
+- `LOG_W` (WARNING):
+    - Something is not in shape, but it's recoverable.
+    - e.g.: A value is not parsable in the config file, which will default.
+- `LOG_M` (MESSAGE):
+    - Important info, which informs about the state.
+    - e.g.: An empty notification does get removed immediately.
+- `LOG_I` (INFO):
+    - Mostly unneccessary info, but important to debug (as the user) some use cases.
+    - e.g.: print the notification contents after arriving
+- `LOG_D` (DEBUG):
+    - Only important during development or tracing some bugs (as the developer).