Include code review details in CONTRIBUTING.md

CONTRIBUTING.md

-Want to contribute? Great! First, read this page (including the small print at the end).
+Want to contribute? Great! First, read this page.
 
 ### Before you contribute
 Before we can use your code, you must sign the
@@ -16,12 +16,45 @@ us first through the issue tracker with your idea so that we can help out and
 possibly guide you. Coordinating up front makes it much easier to avoid
 frustration later on.
 
-### Code reviews
-All submissions, including submissions by project members, require review. We
-use GitHub pull requests for this purpose.
-
-### The small print
 Contributions made by corporations are covered by a different agreement than
 the one above, the
 [Software Grant and Corporate Contributor License Agreement]
 (https://cla.developers.google.com/about/google-corporate).
+
+### Code reviews
+All submissions, including submissions by project members, require review. We
+use GitHub pull requests for this purpose.
+
+### Code Style
+We use Google style guides for all our code. Below are the guidelines for each
+language we use.
+
+#### Go
+All Go source code must be formatted using gofmt and be lint-clean according to
+golint. This will be checked by Travis but can be checked manually from a local
+repo via `make gofmt golint`.
+
+Code is expected to be gofmt- and lint clean before it is submitted for review.
+Code reviews can then focus on structural details and higher level style
+considerations. Many common mistakes are already documented in the
+[Go Code Review Comments](https://github.com/golang/go/wiki/CodeReviewComments)
+doc so it's worth being familiar with these patterns.
+
+#### Python
+All Python source code must be lint-clean according to pylint. This will be
+checked by Travis but can be checked manually from a local repo via
+`make pylint`.
+
+Once code is pylint-clean, it can be submitted for review. In addition to lint
+cleanliness, Python code must adhere to the
+[Google Python Style Guide](https://google.github.io/styleguide/pyguide.html)
+which has a number of additional conventions. Please be familiar with the style
+guide and ensure code satisfies its rules before submitting for review.
+
+##### Borrowed Standard Library Code
+Standard library code that is borrowed from other open source projects such as
+CPython and PyPy need not be lint clean or satisfy the style guide. The goal
+should be to keep the copied sources as close to the originals as possible
+while still being functional. For more details about borrowing this kind of
+code from other places see the
+[guidelines for integration](https://github.com/google/grumpy/wiki/Standard-libraries:-guidelines-for-integration).