added note to README for building HHVM
added note to README for building HHVM

I already made a ticket that I got some errors while building when I noticed it was the missing agent_sdk files (I went straight for the HHVM building instead of just trying the plugin first)
deivisonarthur/hhvm-newrelic-ext,deivisonarthur/hhvm-newrelic-ext,deivisonarthur/hhvm-newrelic-ext,Qoto/hhvm-newrelic-ext,chregu/hhvm-newrelic-ext,Qoto/hhvm-newrelic-ext,effeks/hhvm-newrelic-ext,chregu/hhvm-newrelic-ext,craig-russell/hhvm-newrelic-ext,chregu/hhvm-newrelic-ext,KMK-ONLINE/hhvm-newrelic-ext,effeks/hhvm-newrelic-ext,KMK-ONLINE/hhvm-newrelic-ext,effeks/hhvm-newrelic-ext,craig-russell/hhvm-newrelic-ext,craig-russell/hhvm-newrelic-ext,KMK-ONLINE/hhvm-newrelic-ext,Qoto/hhvm-newrelic-ext