Improve resource provider guide
Improve resource provider guide

* reference to Math global object
* add average accumulation function example
rajkiranrbala/cf-abacus,rajkiranrbala/cf-abacus,rajkiranrbala/cf-abacus,rajkiranrbala/cf-abacus,cloudfoundry-incubator/cf-abacus,cloudfoundry-incubator/cf-abacus,cloudfoundry-incubator/cf-abacus