Added CONTRIBUTING.md (crytic#231)

Author: ggrieco-tob

CONTRIBUTING.md

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+# Contributing to Echidna
+
+First, thanks for your interest in contributing to Echidna! We welcome and
+appreciate all contributions, including bug reports, feature suggestions,
+tutorials/blog posts, and code improvements.
+
+If you're unsure where to start, we recommend to join our [chat room](https://empireslacking.herokuapp.com/)
+(in the #ethereum channel) to discuss new ideas to improve this tool. You can also take a look to the [`help wanted`](https://github.com/crytic/echidna/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22)
+issue labels.
+
+## Bug reports and feature suggestions
+
+Bug reports and feature suggestions can be submitted to our [issue
+tracker](https://github.com/crytic/echidna/issues). For bug reports,
+attaching the contract that caused the bug will help us in debugging and
+resolving the issue quickly. If you find a security
+vulnerability, do not open an issue; email `opensource@trailofbits.com`
+instead.
+
+## Questions
+
+Questions can be submitted to the issue tracker, but you may get a faster
+response if you ask in our [chat room](https://empireslacking.herokuapp.com/)
+(in the #ethereum channel).
+
+## Code
+
+Echidna uses the pull request contribution model. Please make an account on
+Github, fork this repo, and submit code contributions via pull request. For
+more documentation, look [here](https://guides.github.com/activities/forking/).
+
+Some pull request guidelines:
+
+- Minimize irrelevant changes (formatting, whitespace, etc) to code that would
+  otherwise not be touched by this patch. Save formatting or style corrections
+  for a separate pull request that does not make any semantic changes.
+- When possible, large changes should be split up into smaller focused pull
+  requests.
+- Fill out the pull request description with a summary of what your patch does,
+  key changes that have been made, and any further points of discussion, if
+  applicable.
+- Title your pull request with a brief description of what it's changing.
+  "Fixes #123" is a good comment to add to the description, but makes for an
+  unclear title on its own.
+- We use a "CodeClimate" verification check in every PR to make sure the code merged 
+  is tidy and easy to understand.