Add updating instructions

[jonseddon]

In the Installing section of the users' guide add a description of how to update to the latest copy.

[jonseddon]

Something like:

1. In your existing local copy of https://github.com/MetOffice/tenten_tempestextremes, pull in the latest changes with "git pull origin main".
2. In this case I have renamed some of the files and so it would be good to get rid of the old files in the app in your suite, and so in ~/roses/<suite-id> type rm -rf app/tempest_tracker and then type the same in ~/cylc-run/<suite-id> on Linux and the HPC.
3. In the local copy of the repository type cp -r rose-suite/app/tempest_tracker ~/roses/<suite-id>/app/.
4. Then a rose-suite-run --reload will copy the changes to the HPC.