Be explicit that pants requires python 2.7.x to run. In

pantsbuild#1661, John mentioned that pants
can only run in a python 2.7.x interpreter.  So now we try to say so in the
docs.

Reviewed at https://rbcommons.com/s/twitter/r/2343/

README.md

@@ -17,6 +17,7 @@ We use [Coveralls](https://coveralls.io) to monitor test coverage
 At a minimum, pants requires the following to run properly:
 
 * Linux or Mac OS X
-* Python 2.7 or later (2.7.8 recommended)
+* Python 2.7.x (the latest stable version of 2.7 is recommended)
+* A C compiler, system headers, Python headers (to compile native Python modules)
 * OpenJDK 7 or greater, Oracle JDK 6 or greater
 * Internet access (so that pants can fully bootstrap itself)

src/docs/install.md

@@ -18,8 +18,10 @@ there tends to be a lot of "churn".)
 If you're reading this in an organization that already uses Pants, ask
 your neighbor where your source code is.
 
-There are a few ways to get a runnable version of Pants into a
-developer's workspace.
+There are a few ways to get a runnable version of Pants into a developer's workspace. Before
+beginning, please [consult the README](https://github.com/pantsbuild/pants/blob/master/README.md),
+to make sure your machine fits the requirements. In particular, you'll want to make sure that you
+have Python 2.7.x -- pants itself needs to be hosted on that version.
 
 Virtualenv-based Installation
 -----------------------------