Clean up CONTRIBUTING.md: (#5911)

- Reorganize sections for better navigability
- Add table of contents
- Rework frontend assets section for clarity and DRY
- Rework translating section for clarity, add "Add Translations" contribution type
- Move release docs only useful for maintainers to RELEASING.md
- Other miscellaneous improvements

RELEASING.md

+## Refresh documentation website
+
+Every once in a while we want to compile the documentation and publish it.
+Here's how to do it.
+
+```bash
+# install doc dependencies
+pip install -r docs/requirements.txt
+
+# build the docs
+python setup.py build_sphinx
+
+# copy html files to temp folder
+cp -r docs/_build/html/ /tmp/tmp_superset_docs/
+
+# clone the docs repo
+cd ~/
+git clone https://git-wip-us.apache.org/repos/asf/incubator-superset-site.git
+
+# copy
+cp -r /tmp/tmp_superset_docs/ ~/incubator-superset-site.git/
+
+# commit and push to `asf-site` branch
+cd ~/incubator-superset-site.git/
+git checkout asf-site
+git add .
+git commit -a -m "New doc version"
+git push origin master
+```
+
+## Publishing a PyPI release
+
+We create a branch that goes along each minor release `0.24`
+and micro releases get corresponding tags as in `0.24.0`. Git history should
+never be altered in release branches.
+Bug fixes and security-related patches get cherry-picked
+(usually from master) as in `git cherry-pick -x {SHA}`.
+
+Following a set of cherries being picked, a release can be pushed to
+PyPI as follows:
+
+```bash
+# branching off of master
+git checkout -b 0.25
+
+# cherry-picking a SHA
+git cherry-pick -x f9d85bd2e1fd9bc233d19c76bed09467522b968a
+# repeat with other SHAs, don't forget the -x
+
+# source of thruth for release numbers live in package.json
+vi superset/assets/package.json
+# hard code release in file, commit to the release branch
+git commit -a -m "0.25.0"
+
+# create the release tag in the release branch
+git tag 0.25.0
+git push apache 0.25 --tags
+
+# check travis to confirm the build succeeded as
+# you shouldn't assume that a clean cherry will be clean
+# when landing on a new sundae
+
+# compile the JS, and push to pypi
+# to run this part you'll need a pypi account and rights on the
+# superset package. Committers that want to ship releases
+# should have this access.
+# You'll also need a `.pypirc` as specified here:
+# http://peterdowns.com/posts/first-time-with-pypi.html
+./pypi_push.sh
+
+# publish an update to the CHANGELOG.md for the right version range
+# looking the latest CHANGELOG entry for the second argument
+./gen_changelog.sh 0.22.1 0.25.0
+# this will overwrite the CHANGELOG.md with only the version range
+# so you'll want to copy paste that on top of the previous CHANGELOG.md
+# open a PR against `master`
+```
+
+In the future we'll start publishing release candidates for minor releases
+only, but typically not for micro release.
+The process will be similar to the process described above, expect the
+tags will be formatted `0.25.0rc1`, `0.25.0rc2`, ..., until consensus
+is reached.
+
+We should also have a Github PR label process to target the proper
+release, and tooling helping keeping track of all the cherries and
+target versions.
+
+For Apache releases, the process will be a bit heavier and should get
+documented here. There will be extra steps for signing the binaries,
+with a PGP key and providing MD5, Apache voting, as well as
+publishing to Apache's SVN repository. View the ASF docs for more
+information.

docs/installation.rst

@@ -672,20 +672,12 @@ and run via: ::
 
     celery flower --app=superset.sql_lab:celery_app
 
-Making your own build
+Building from source
 ---------------------
 
-For more advanced users, you may want to build Superset from sources. That
+More advanced users may want to build Superset from sources. That
 would be the case if you fork the project to add features specific to
-your environment.::
-
-    # assuming $SUPERSET_HOME as the root of the repo
-    cd $SUPERSET_HOME/superset/assets
-    yarn
-    yarn run build
-    cd $SUPERSET_HOME
-    python setup.py install
-
+your environment. See `CONTRIBUTING.md <https://github.com/apache/incubator-superset/blob/master/CONTRIBUTING.md#local-development>`_.
 
 Blueprints
 ----------

superset/translations/requirements.txt

+Babel==2.5.3