[DOC] Describe what bids-matlab can and cannot do

[Remi-Gau]

This should fix #77.

Main change

This adds a very generic description of what the toolbox can do and what functions do it, as this was pointed out as one of the obvious missing point in documentation.

Minor change

Adds some details about our "philosophy" of what Octave / Matlab version we support.

---

For reviewers

• Is this clear enough?
• Is is an issue that this points at some functions that should be more for "developpers" than for "users", as we would like the latter to mostly use layout, query, report and not the internal functions.
• Anything missing?

[gllmflndn]

Thanks for this, @Remi-Gau!

[apjanke]

Suggested change

	We aim for compatibility with the latest stable release of Octave at any time. Compatibility can sometimes also be achieved with older versions of Octave but this is not guaranteed.
	We aim for compatibility with the latest stable major version of Octave at any time. Compatibility can sometimes also be achieved with older versions of Octave but this is not guaranteed.

Maybe you should declare support for a full major version series like Octave 5.x instead of just the "latest stable release", which could be interpreted as "exactly 5.2.3" or similar.

Maybe declare a policy about how long Octave major versions are supported? For example, Octave 6.x is right around the corner. How long will you support Octave 5.x once 6.x is out?

And what about a policy for how long older Matlab versions are supported? If you support R2014a forever, you're eventually going to have a lot of Matlab versions to test against, and you'll never get to adopt newer Matlab features.