Clarify aligning with toolchain a bit more

Author: sdboyer

CONTRIBUTING.md

@@ -61,6 +61,9 @@ again.
 * _Be kind, respectful, and inclusive_. Really live [that CoC](https://github.com/golang/dep/blob/master/CODE_OF_CONDUCT.md). We've developed a reputation as one of the most welcoming and supportive project environments in the Go community, and we want to keep that up!
 * The lines of responsibility between maintainership areas can be fuzzy. Get to know your fellow maintainers - it's important to work _with_ them when an issue falls in this grey area.
 * Remember, the long-term goal of `dep` is to disappear into the `go` toolchain. That's going to be a challenging process, no matter what. Minimizing that eventual difficulty should be a guiding light for all your decisions today.
+  * Try to match the toolchain's assumptions as closely as possible ([example](https://github.com/golang/dep/issues/564#issuecomment-300994599)), and avoid introducing new rules the toolchain would later have to incorporate.
+  * Every new flag or option in the metadata files is more exposed surface area that demands conversion later. Only add these with a clear design plan.
+  * `dep` is experimental, but increasingly only on a larger scale. Experiments need clear hypotheses and parameters for testing - nothing off-the-cuff.
 * Being a maintainer doesn't mean you're always right. Admitting when you've made a mistake keeps the code flowing, the environment health, and the respect level up.
 * It's fine if you need to step back from maintainership responsibilities - just, please, don't fade away! Let other maintainers know what's going on.