add a release guide

Even though there are very few steps involved, it is good to have it documented.

RELEASE.md

+# Making a Adaptive release
+
+This document guides a contributor through creating a release of Adaptive.
+
+
+## Preflight checks
+
+The following checks should be made *before* tagging the release.
+
+
+#### Check that all issues are resolved
+
+Check that all the issues and merge requests for the appropriate
+[milestone](https://gitlab.kwant-project.org/qt/adaptive/issues)
+have been resolved. Any unresolved issues should have their milestone
+bumped.
+
+
+#### Ensure that all tests pass
+
+For major and minor releases we will be tagging the ``master`` branch.
+This should be as simple as verifying that the 
+[latest CI pipeline](https://gitlab.kwant-project.org/qt/adaptive/pipelines) 
+succeeded.
+
+
+#### Verify that `AUTHORS.md` is up-to-date
+
+The following command shows the number of commits per author since the last
+annotated tag:
+```
+t=$(git describe --abbrev=0); echo Commits since $t; git shortlog -s $t..
+```
+
+## Make a release, but do not publish it yet
+
+
+### Tag the release
+
+Make an **annotated, signed** tag for the release. The tag must have the name:
+```
+git tag -s v<version> -m "version <version>"
+```
+
+
+### Build a source tarball and wheels and test it
+
+```
+rm -fr build dist
+python setup.py sdist bdist_wheel
+```
+
+This creates the file `dist/adaptive-<version>.tar.gz`.  It is a good idea to unpack it 
+and check that the tests run:
+```
+tar xzf dist/adaptive*.tar.gz
+cd adaptive-*
+py.test .
+```
+
+## Upload to PyPI
+
+```
+twine upload dist/*
+```
+
+
+## Update the [conda-forge recipe](https://github.com/conda-forge/adaptive-feedstock)
+
+* Fork the [feedstock repo](https://github.com/conda-forge/adaptive-feedstock)
+* Change the version number and sha256 in `recipe/meta.yaml` and commit to your fork
+* Open a [Pull Request](https://github.com/conda-forge/adaptive-feedstock/compare)
+* Type `@conda-forge-admin, please rerender` as a comment
+* When the tests succeed, merge