Document the semantics of OpenMode

[jorisdral]

First mentioned here: #85 (comment)

The semantics of OpenMode are not properly documented anywhere. For example, opening a file in read mode should/will throw an error if the file does not exist, but this is not documented. More importantly, it is not documented whether files can be opened multiple times in different modes: for example, can a file be opened in both read- and write-mode?