[docs] Little explanation for test/core directory

test/core/README.md

+# `test/core`
+
+`test/core` directory is a place where tests from node.js core go. They are
+here to ensure that node-http-proxy works just fine with all kinds of
+different situations, which are covered in core tests, but are not covered in
+our tests.
+
+All these tests require little modifications to make them test node-http-proxy,
+but we try to keep them as vanilla as possible.
+