Document process of upgrading openbalena instance

[balena-ci]

We should document how a user can update their openbalena instance from one release to the next.

[balena-ci]

[shaunmulligan] This issue has attached support thread https://jel.ly.fish/#/support-thread~e7759e37-89e2-47fa-b63e-40330e09a8c0

[pdonorio]

Hello! Any update on this? The support thread is not visible, can anyone paste the relative steps at least?

Thank you

[dfunckt]

The current approach is to maintain a checkout of openBalena and regularly update it with git pull. All instance-specific configuration -- i.e. the config directory and Docker volumes -- will not be touched. Do check the project repository here on Github beforehand however, because there may be breaking changes you should be aware of.