Add DEPLOY.md with instructions for deploying to PyPI

DEPLOY.md

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+# Deployment instructions for PyPi
+
+This file is meant for maintainers of pycapnp, and documents the process for uploading to PyPI.
+
+## Pre-requisites
+
+```
+pip install pypandoc cython
+```
+
+## Run tests
+
+I typically sanity check by running the tests once again locally, but as long as Travis is green, you're probably fine.
+
+## Add a commit that bumps the version
+
+Bump the version in setup.py, and add descriptions of all the changes to CHANGELOG.md (see 19e1b189caa786c7f572e679d6bb94aadfbdb5e0 for an example commit).
+
+## Run the build and upload
+
+Run the following command to clean up old artifacts, run the build, and then upload the result to PyPI
+
+```
+rm -rf bundled/ capnp/version.py capnp/lib/capnp.{h,cpp} build; python setup.py build && python setup.py sdist upload -r PyPI
+```
+
+## Test the PyPI release
+
+I manually test the PyPI release after it's been uploaded. I have a few virtualenvs that I manually run the following command in (run this from the pycapnp directory since it runs the tests at the end):
+```
+yes | pip uninstall pycapnp; pip install pycapnp && py.test test
+```
+
+I usually test the following configurations:
+- Python 2.7 with and without cython installed
+- Python 3.6 with and without cython installed
+
+This step could probably benefit greatly from some automation. Perhaps even Travis could handle it, but I'm not sure how best to trigger Travis from a PyPI release.