Add usage instructions to README and help.

[MicahZoltu]

I build/installed evmdis and ran evmdis -h. It shows me flags I can pass but doesn't actually tell me how to use it. I took a guess and did:
echo <some evm bytecode> | evmdis but that resulted in:

# Stack: []
0x0     STOP()

Which doesn't seem correct. Readme should have usage instructions, and the CLI help should as well.

[Arachnid]

Yes, usability needs big improvements. For reference, try echo -n; one of the many current shortcomings is very picky hex parsing.

[bohendo]

If anyone is analyzing an output file that might contain a newline, you can also do the following:

cat file.bytecode | tr -d '\n' | evmdis

(tr is a unix command line tool, tr for translate, -d for delete aka translate all '\n' chars into nothing)