Update docs to cover BIG_ENDIAN

[ajrudzitis]

I don't see configuring endian-ness to be covered in the documentation. I found it in the examples, but it would be useful to add it to the documentation explicitly.

[y1ca1]

Good point, we will add that information somewhere. Also out of curiosity, do you feel the need to specify endianness at finer granularity (e.g., per format definition instead of per .vest file)? We can consider adding that feature as well.

[ajrudzitis]

I have not run into the case of needing to specify it per format definition, but I think it is reasonable that I or someone, someday will have that requirement.