Documentation: msg should be msghash

[bitsanity]

It looks like the ecdsaSign() function expects the hash of a message instead of the message itself.
The Usage section could be clearer:

// msghash should be 32-byte hash of a message, e.g. sha256(msg)
// here using random data just for illustration and test
const msghash32 = randomBytes(32)

[snip]

// sign the message
const sigObj = secp256k1.ecdsaSign(msghash32, privKey)

This would also fix #132

[fanatid]

Yep, would be good. Do you want submit PR?

[bitsanity]

Attached patch file (had to append .txt because github doesn't recognize that file type):

fix_issue_163.patch.txt

[fanatid]

Added in d9f7bf0