Adding contribution note

Signed-off-by: Gerald Venzl <gerald.venzl@gmail.com>

CONTRIBUTING.md

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+# Contributing
+Oracle welcomes contributions to this repository from anyone.
+
+If you want to submit a pull request to fix a bug or enhance an existing
+`Vagrantfile`, please first open an issue and link to that issue when you
+submit your pull request.
+
+If you have any questions about a possible submission, feel free to open
+an issue too.
+
+## Contributing to the Oracle Vagrant Boxes repository
+
+Pull requests can be made under
+[The Oracle Contributor Agreement](https://www.oracle.com/technetwork/community/oca-486395.html) (OCA).
+
+For pull requests to be accepted, the bottom of your commit message must have
+the following line using your name and e-mail address as it appears in the
+OCA Signatories list.
+
+```
+Signed-off-by: Your Name <you@example.org>
+```
+
+This can be automatically added to pull requests by committing with:
+
+```
+git commit --signoff
+```
+
+Only pull requests from committers that can be verified as having
+signed the OCA can be accepted.
+
+### Pull request process
+
+1. Fork this repository
+1. Create a branch in your fork to implement the changes. We recommend using
+the issue number as part of your branch name, e.g. `1234-fixes`
+1. Ensure that any documentation is updated with the changes that are required
+by your fix.
+1. Ensure that any samples are updated if the base image has been changed.
+1. Submit the pull request. *Do not leave the pull request blank*. Explain exactly
+what your changes are meant to do and provide simple steps on how to validate
+your changes. Ensure that you reference the issue you created as well.
+We will assign the pull request to 2-3 people for review before it is merged.
+
+
+*Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.*