README changes to AES-GCM comments

[khrice]

Changed the comment to help with confusion around optional parameters for IE and Edge

[Ruffio]

@khrice nice catch, you are absolutely right :-)

[ericlaw1979]

It might also be good to mention that IE/Edge Legacy require that the IV be 12 bytes for AES_GCM; any other value results in failure. This is because these browsers all the Windows Crypto API which supports only a 96bit IV (as suggested by the NIST standard).