libbase: add a README.md covering the most frequent question.

Test: N/A
Change-Id: Iebea3e74bb7d59778fa0d2342e51bb4ffc5450a3

base/README.md

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+# libbase
+
+## Who is this library for?
+
+This library is a collection of convenience functions to make common tasks
+easier and less error-prone.
+
+In this context, "error-prone" covers both "hard to do correctly" and
+"hard to do with good performance", but as a general purpose library,
+libbase's primary focus is on making it easier to do things easily and
+correctly when a compromise has to be made between "simplest API" on the
+one hand and "fastest implementation" on the other. Though obviously
+the ideal is to have both.
+
+## Should my routine be added?
+
+The intention is to cover the 80% use cases, not be all things to all users.
+
+If you have a routine that's really useful in your project,
+congratulations. But that doesn't mean it should be here rather than
+just in your project.
+
+The question for libbase is "should everyone be doing this?"/"does this
+make everyone's code cleaner/safer?". Historically we've considered the
+bar for inclusion to be "are there at least three *unrelated* projects
+that would be cleaned up by doing so".
+
+If your routine is actually something from a future C++ standard (that
+isn't yet in libc++), or it's widely used in another library, that helps
+show that there's precedent. Being able to say "so-and-so has used this
+API for n years" is a good way to reduce concerns about API choices.
+
+## Any other restrictions?
+
+Unlike most Android code, code in libbase has to build for Mac and
+Windows too.
+
+Code here is also expected to have good test coverage.
+
+By its nature, it's difficult to change libbase API. It's often best
+to start using your routine just in your project, and let it "graduate"
+after you're certain that the API is solid.