Add contributing and security markdown files

Author: sallain

CONTRIBUTING.md

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+# Contributing
+
+This repository is the place to file a3m bug reports as well as make suggestions
+for new or enhanced features. Anyone with a GitHub account can add an issue,
+comment on someone else's issue, or make a pull request.
+
+## Security
+
+If you have a security concern about a3m or any of its companion repositories,
+please do not file it here. See the [security policy](SECURITY.md) in this
+repository for directions on how to report security issues.
+
+## Filing an issue
+
+All changes to a3m should start with an issue, including bug fixes, new
+features, and enhancements to existing features.
+
+To file an issue, go to [the Issues
+tab](https://github.com/artefactual-labs/a3m/issues) and click the green **New
+issue** button in the top right-hand corner. You can select the appropriate
+template from the list. Fill out the template with as much information as you
+can.
+
+An issue should describe a behaviour without implying a solution. The pull
+request that may follow, if changes to the codebase are necessary, fixes the
+problem. Framing your issue as a problem statement helps everyone understand why
+the issue is important - it describes how Enduro is not performing as it should
+(bug) or as it could (enhancement). Please title your issue as a problem
+statement, starting with "Problem:". You can check [existing
+issues](https://github.com/archivematica/Issues/issues) for examples.
+
+### Reporting a bug
+
+To report a bug, select **Bug report** from the issue templates and fill out the
+fields with as much information as possible.
+
+Useful information to provide includes:
+
+* What version of a3m are you using?
+* Are you using it with Enduro?
+* How was it installed?
+* Was this a fresh install or an upgrade?
+* What did you do to cause this bug to happen?
+* What did you expect to happen?
+* What did you see instead?
+* Can you reproduce this reliably?
+
+### Submitting an enhancement idea
+
+To suggest a new feature or an enhancement to an existing feature, select
+**Feature request** from the issue templates and fill out the fields with as
+much information as possible.