DOC: mention explicitly text files as media files

[hagenw]

Closes #439

Rephrases the README and data format specifications to better indicate that media files are not restricted in its file type.

In addition, it explicitly names text files as potential media files. I also state that a segmented table can be used, if applicable, as some file types might not have a direct connection to a timeline. E.g. you can create a segmented table for text files, but it might be meaningless as no other application might be able to use that information.

README

Format specifications -- Introduction

Format specification -- Database

[ChristianGeng]

This updates the documentation to reflect that it generalizes "audio" data to more general "media" data.

The only concern that I have is whether even "media" is general enough, or whether even a more general term like "sensor data" should be used, as future releases might need to expand to other sensing modalities with fixed sampling rate - or even rare events that are not sampled.

Still, the description is valid for now and therefore approval will be given.

[hagenw]

Thanks for pointing out the concern regarding "Media file". I created #444 to track that, as I also think this would require more rewriting of the documentation, and should be handled independent of this pull request.