Added info about bytes 0x78 0x9c

[ResaloliPT]

First of all, thank you very much for the awsome readme, it was a enormous help understanding UE4 paks.

Now, under 3.4 Data record on the readme there's a note relating to the 0x78 0x9c, since i´m making a C# implementation on my end.
I've learned what those bytes are, they are part of the zLib header (declaring compression level 6 which is the default).

More about that can be found here https://yal.cc/cs-deflatestream-zlib/

Each chunk located in file data inside a Data record will be the following format 2 bytes for the zLib header (0x78 0x9c normaly) then the compressed data and finally 2 bytes that contain a Adler-32 checksum of the compressed data.

Can also share:
My C# Implementation of V3 pak

zLib chunk compressor