Document why changing some characters in a Base64-encoded string does not invalidate signatures

[lhazlewood]

Add a section to the Readme why this doesn't invalidate signature calculation and is not a flaw in the library. See #211 #269 #324 and others.

Also:

#520 (comment)

https://stackoverflow.com/questions/29941270/why-do-base64-decode-produce-same-byte-array-for-different-strings

https://stackoverflow.com/questions/33663113/multiple-strings-base64-decoded-to-same-byte-array?noredirect=1&lq=1