NPM install missing colordiff dependency

[diamondman]

➜  ~ npm install -g pdiffjson
added 1 package, and audited 2 packages in 947ms
found 0 vulnerabilities
➜  ~ pdiffjson ~/Documents/modem.list ~/Documents/modem_new.list 
~/.nvm/versions/node/v16.4.0/bin/pdiffjson: line 56: colordiff: command not found
➜  ~ npm install -g colordiff                                        
added 9 packages, and audited 10 packages in 922ms
found 0 vulnerabilities
➜  ~ pdiffjson ~/Documents/modem.list ~/Documents/modem_new.list //WORKS

[jlevy]

I'd assumed jq and colordiff are system (not npm) installations but that's another approach worth considering.

For now clarified the readme.