Update README.md

[Dmole]

7z

[vasi]

Is 7z a filter compressor (like pixz, gzip, ztsd...) or an archive format (like zip?) If the latter, I'm not sure it makes sense to attempt to compare directly.

[Dmole]

Igor made the first implementation of LZMA with 7z in 1999.

If pixz is a fix for "random access" of ".xz file format...tarballs" then it should be noted why 7z which has the same feature and is what xz was derived from is not used.

It's not possible for pixz to perform "random access" on a stream so while it may be a filter compressor what is the use case? (and why is the README talking about files if the use case is filter/stream related)