Add alien.py debugging page

docs/user/alienpy_debug.md

+# alien.py Debugging
+
+There are a few steps to follow
+
+### Report the version
+`alien.py version`
+
+will provide an output like:
+```
+alien.py version: 1.3.7
+alien.py version date: 20211202_153804
+alien.py location: /cvmfs/alice.cern.ch/el7-x86_64/Packages/xjalienfs/1.3.7-19/lib/python3.6/site-packages/alienpy/alien.py
+script location: /cvmfs/alice.cern.ch/el7-x86_64/Packages/xjalienfs/1.3.7-19/bin/alien.py
+Interpreter: /cvmfs/alice.cern.ch/el7-x86_64/Packages/Python/v3.6.10-149/bin/python3.6
+Python version: 3.6.10 (default, Mar 17 2022, 14:52:02) 
+[GCC 10.2.0]
+XRootD version: 5.4.2-alice1
+XRootD path: /cvmfs/alice.cern.ch/el7-x86_64/Packages/XRootD/v5.4.2-alice1-3/lib/python/site-packages/XRootD/client/__init__.py
+```
+
+this is not network dependent so it can be executed at any time
+
+### alien.py debug report
+
+Enabling debug output  
+`ALIENPY_DEBUG=1 ALIENPY_DEBUG_FILE=log.txt`   
+    
+* `ALIENPY_DEBUG` - will enable the debug messages   
+* `ALIENPY_DEBUG_FILE` - allow specification of output logfile. in the example above `log.txt` will be placed in `CWD`   
+
+### XRootD debug report
+
+Enabling XRootD system log output
+`XRD_LOGLEVEL='Dump' XRD_LOGFILE=xrdlog.txt`   
+
+* `XRD_LOGLEVEL='Dump'` - will enable/change the log level; `Dump` is the highest
+* `XRD_LOGFILE=xrdlog.txt` - will specify the location of the log file.
+
+Additionally for checking the downloading with `xrdcp` one can enable   
+the persistence of metafile(s) used for downloading:   
+`ALIENPY_KEEP_META=1`
+
+this will copy in the `CWD` the metafiles used for downloading, which can be used with xrdcp:   
+`xrdcp -p -P -f -v meta_file_name.meta4 destination_file`
+
+#### XRootD python import
+
+For problems related to python loading of XRootD binding one can check with:   
+`python3 -c 'from XRootD import client;print(client.__version__);'`
+
+and if this is valid, the actual location of library can be checked with:   
+`python3 -c 'from XRootD import client;print(client.__file__);'`
+
+if the loading fails, one can ask on [alice-project-analysis-task-force@cern.ch](mailto:alice-project-analysis-task-force@cern.ch)   
+or    
+fill an issue on alidist repository [https://github.com/alisw/alidist/issues](https://github.com/alisw/alidist/issues)
+
+
+### Developer contact
+Adrian Sevcenco [adrian.sevcenco@cern.ch](mailto:adrian.sevcenco@cern.ch)
+or
+on mattermost `JAliEn` channel: [alice/channels/jalien](https://mattermost.web.cern.ch/alice/channels/jalien)   
+or mattermost direct message: `@asevcenc`
+
+There is also a possibility for discussions on [ALICE Talk](https://alice-talk.web.cern.ch/)   
+but i have to be made aware of discussion so please mention me
+
+