contributing instructions added

CONTRIBUTING.md

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+# Pyrocko development environment and contribution guide
+
+## Language
+
+Pyrocko is written in the Python programming language (versions ==2.7 and >=3.4).
+
+## Deployment
+
+Pyrocko uses Setuptools for its installation script. See `setup.py` and
+`setup.cfg` in the project root directory.
+
+## Versioning and releases
+
+Git is used for version control. Use development branches for new features.
+Master branch should always point to a stable version.
+
+The Pyrocko project  adheres to a [calendar dating versioning scheme](https://calver.org/#scheme), using the vYYYY.MM.DD format.
+
+Notable changes must be documented in the file `CHANGELOG.md`. The format of
+the change log is based on [Keep a
+Changelog](https://keepachangelog.com/en/1.0.0/).
+
+### Commit message conventions
+
+* start with lower case
+* colon-prepend affected component
+* try to use imperative form
+* examples:
+  - `docs: add section about weighting`
+  - `waveform targets: correct typo in component names`
+  - `waveform targets: fix issues with misaligned traces`
+
+### Branching policy
+
+* The `master` branch should point to a stable, tagged version of Pyrocko.
+* The `dev` branch is used to aggregate new features before releasing.
+* Use topic branches to develop new features.
+
+### Rebase small changes before pushing
+
+Try to rebase little changes on top of master (or any other development branch)
+before pushing, it makes the history much better readable. Here is a safe way
+to do so.
+
+*If we have already commited and merged changes to local master:*
+
+```sh
+git checkout master
+git fetch origin    # important, otherwise we rebase to outdated
+git rebase origin/master
+git push origin master
+```
+
+with `git config --global pull.rebase true` this can be shortcutted to
+
+```sh
+git pull
+```
+
+*Or after we have commited to a feature branch:*
+
+```sh
+git checkout feature
+git fetch origin
+git rebase origin/master
+git checkout master
+git merge origin/master
+git merge feature    # should now be fast forward...
+git push origin master
+```
+
+If during push it refuses to upload ('not fast forward...') then repeat the
+procedure, because someone else has pushed between your fetch and push.
+
+**Tip:** use `rebase -i ...` to simplify/fixup/beautify your changeset.
+
+## Code style
+
+Pyrocko source code must follow the PEP8 coding standards. It must pass the
+code style check provided by the `flake8` tool.
+
+Additionally,
+
+* use i/n convention for indices and counts
+  - e.g. `for istation in range(nstations):`
+* do not abbreviate words unless this would result in ridiculously long names
+* use British english, e.g.
+  - 'modelling' rather than 'modeling'
+  - 'analyser' rather than 'analyzer'
+  - 'optimiser' rather than 'optimizer'
+* log and exception messages:
+  - capital beginning
+  - final period
+  - Progress actions should end with `...`, e.g. `Generating report's archive...`
+  - e.g. `raise ProblemDataNotAvailable('No problem data available (%s).' % dirname)`
+  - in-text names must be quoted; not needed after colons
+* docstrings: TODO
+
+## Documentation
+
+Pyrocko's documentation is built using the `Sphinx` tool. See the `docs`
+in the project root directory. Build with `make html` in `docs`.
+
+*Text style rules:*
+
+* titles: only capitalize first word
+* use British english
+
+## License
+
+GNU General Public License, Version 3, 29 June 2007
+
+Copyright © 2018 Helmholtz Centre Potsdam GFZ German Research Centre for
+Geosciences, Potsdam, Germany and University of Kiel, Kiel, Germany.
+
+Pyrocko is free software: you can redistribute it and/or modify it under the
+terms of the GNU General Public License as published by the Free Software
+Foundation, either version 3 of the License, or (at your option) any later
+version. Pyrocko is distributed in the hope that it will be useful, but WITHOUT
+ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+You should have received a copy of the GNU General Public License along with
+this program. If not, see <http://www.gnu.org/licenses/>.
+