Document one-way sync

[piater]

Vdirsyncer supports one-way (unidirectional) sync as described in #468. However, this is not obvious in the documentation. It actually took me a while to find it and verify that it works by implementing a new unit test. Here is a suggestion: In Syncing with read-only storages, replace the paragraph

You may get [...] rights to do so.

by the following text:

You may get into a situation where you want to delete, modify or add some events in your holidays calendar. If you try to do that, you’ll notice that vdirsyncer will revert any changes you’ve made after a few times of running sync, restoring deleted events, reverting changed events, and deleting added events. This is because vdirsyncer wants to keep everything in sync, and it can’t synchronize changes to the public holidays calendar because it doesn’t have the rights to do so.

Also in the manual, it would be good to document that partial_sync = "ignore" prevents items in B but not in A from being erased during synchronization.