List required tools to build from sources (#476)
List required tools to build from sources (#476)

* List required tools to build from sources

Adds the list of required tools to build from sources and so other minor changes to CONTRIBUTING.md

* Add note about build requiring $GOPATH/bin on $PATH

open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib,census-instrumentation/opencensus-service,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib,census-instrumentation/opencensus-service