Add explanatory language about the "a-format" files

[mtmccrea]

From @elifieldsteel's recent PR (#5)

It may be helpful if specific information about files in the "b-format" folder were included in the README file (specifically to do with Ambisonic order, component ordering, normalization, and encoding radius), to make the converting/decoding process as unambiguous as possible. Some language is suggested in this PR as a starting point, feel free to reword it, move it elsewhere in the README, etc.

These changes were merged, but still TODO:

It might also be appropriate to add similar language about the "a-format" files, since there may be even more ambiguity here regarding the tetrahedral orientation and conversion/decoding process. Proposed language for these files is not included here, because I'm not fully aware of these specific details.