Add a word about php gmp requirement

[SofienM]

Hi,
While trying to install jaeger, I ran multiple times on this error

Exception has occurred.
Error: Call to undefined function OpenCensus\Trace\Exporter\Jaeger\gmp_add()

I think it will be nice to add to the README a word about needing to install GMP.

Anyway thanks for your work and your contributions and if you agree, I can submit a small pull requests for the README.