doc: Adding help for core benchmark in jenkins

PR-URL: #144
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

docs/core_benchmarks.md

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+# How to: Running core benchmarks on Node.js CI
+
+## Current function
+The current jenkins job aims at allowing the impact of a PR on a particular branch to be compared. For example how does PR XXXX impact performance on master?
+
+## How to configure the job
+To start with visit https://ci.nodejs.org/job/benchmark-node-micro-benchmarks/build?delay=0sec
+Here you have four values to enter:
+### BRANCH
+This is which branch or tag to compare to, defaults to master
+### PULL_ID
+Pull request to test
+These two variables provide the two copies of Node.js to compare - we build BRANCH, then take a copy and apply the diff of PULL_ID, and build again. We now have two copies of Node to compare
+
+### CATEGORY
+This is a category of benchmarks in the node/benchmark folder from core
+### FILTER
+This reduces further the benchmarks in scope and is includes benchmark where FILTER exists in the filename.
+
+On running, a minimum of 30 iterations of each benchmark/variation per build, once they have all finished Rscript will summarise the results, suggesting improvement / regression as well as confidence intervals.