linking default value

[olsonpm]

No description provided.

[dougwilson]

Z_DEFAULT_COMPRESSION requests a default compromise between speed and compression (currently equivalent to level 6).

We can just add that directly into the readme, rather than a esoteric link :)

[Fishrock123]

Hmmm, I thought it defaulted to level 7?

[dougwilson]

Hmmm, I thought it defaulted to level 7?

Maybe. I only took it from the linked documentation, which may or may not apply to the bundled version of zlib in Node.js. It's easy to test: compress something with -1, then compress it with 6. Are they the same bytes? Then that's the default :)

[Fishrock123]

Hmm, looks like it should be 6. My bad.

[olsonpm]

Yeah, i didn't include the number because i thought that was the only reason to
disclude it in the first place.

[jonathanong]

can*