unzip on Linux corrupts .img

[tuxlifan]

Hello,

Please add a checksum for the stereopi-opencv-20190405.zip file
(sha256: 6c7a5714db03aae07aeb009483f3fcd0fa43356280da071fdc3bad31dfe6f0c4
md5: fa82c9b997f6797c8445602b933b6ae4)
to the README and a warning for Linux users to NOT use unzip to extract the zip file but use "7z e stereopi-opencv-20190405.zip" instead.

md5sum using unzip (and getting an error message about "Bad CRC"): 3ce7a63479ffa65fc923297d1fa01792
md5sum using 7z:
8dbe549e3a467a583a961bad85e45ca1

Thanks :)

[realizator]

@tuxlifan Thank you! I've updated readme.
I will try to use another ZIP method in the future images!