write some docs

Author: antocuni

README.md

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+manylinux-pypy
+===============
+
+.. image:: https://travis-ci.org/antocuni/manylinux-pypy.svg?branch=master
+    :target: https://travis-ci.org/antocuni/manylinux-pypy
+
+Docker image for building PyPy manylinux_ wheels.
+
+This image extends the official manylinux image by installing PyPy. For now,
+the only supported platform is ``x86_64``.
+
+.. manylinux: https://github.com/pypa/manylinux
+
+At the moment of writing, this image provide the two following versions of
+PyPy:
+
+  - PyPy2.7 7.1.1
+
+  - PyPy3.6 7.1.1
+
+The various PyPy versions are installed inside ``/opt/pypy/``, and they are
+also symlinked to ``/opt/python``. Moreover, each installation of PyPy
+contains also a ``python`` symlink; all the following commands are equivalent
+and run the PyPy interpreter, you can use whatever fits best in your build
+system:
+
+  - ``/opt/pypy/pypy2.7-7.1.1/bin/pypy``
+
+  - ``/opt/pypy/pypy2.7-7.1.1/bin/python``
+
+  - ``/opt/python/pp271-pypy_41/bin/pypy``
+
+  - ``/opt/python/pp271-pypy_41/bin/python``