Add PACELC theorem explanation near CAP theorem

[seonWKim]

Because of some limitation of CAP theorem(which is related to partition tolerance), there is another theorem called PACELC theorem. It deals with real world issues and I think it might be great to add some information about PACELC. Let me know it there's some room I can contribute.

https://en.wikipedia.org/wiki/PACELC_theorem