Add contributing guidelines

CONTRIBUTING.md

+# How to contribute
+
+Contributors are essential to Scapy (as they are to most open source
+projects). Here is some advice to help you help the project!
+
+## Project objectives
+
+We try to keep Scapy as powerful as possible, to support as many
+protocols and platforms as possible, to keep and make the code (and
+the commit history) as clean as possible.
+
+Since Scapy can be slow and memory consuming, we try to limit CPU and
+memory usage, particularly in parts of the code often called.
+
+## Reporting issues
+
+### Questions
+
+It is OK so submit issues to ask questions (more than OK,
+encouraged). There is a label "question" that you can use for that.
+
+### Bugs
+
+If you have installed Scapy through a package manager (from your Linux
+or BSD system, from PyPI, etc.), please get and install the current
+development code, and check that the bug still exists before
+submitting an issue.
+
+Please label your issues "bug".
+
+If you're not sure whether a behavior is a bug or not, submit an issue
+and ask, don't be shy!
+
+### Enhancements / feature requests
+
+If you want a feature in Scapy, but cannot implement it yourself or
+want some hints on how to do that, open an issue with label
+"enhancement".
+
+Explain if possible the API you would like to have (e.g., give examples
+of function calls, packet creations, etc.).
+
+## Submitting pull requests
+
+### Coding style & conventions
+
+First, Scapy "legacy" code contains a lot of code that do not comply
+with the following recommendations, but we try to comply with the some
+guidelines for new code.
+
+  - The code should be PEP-8 compliant; you can check your code with
+    [pep8](https://pypi.python.org/pypi/pep8).
+  - [Pylint](http://www.pylint.org/) can help you write good Python
+    code (even if respecting Pylint rules is sometimes either too hard
+    or even undesirable; human brain needed!).
+  - [Google Python Style Guide](https://google.github.io/styleguide/pyguide.html)
+    is a nice read!
+  - Avoid creating unnecessary `list` objects, particularly if they
+    can be huge (e.g., when possible, use `xrange()` instead of
+    `range()`, `for line in fdesc` instead of `for line in
+    fdesc.readlines()`; more generally prefer generators over lists).
+
+### Tests
+
+Please consider adding tests for your new features or that trigger the
+bug you are fixing. This will prevent a regression from being
+unnoticed.
+
+### New protocols
+
+New protocols can go either in `scapy/layers` or to
+`scapy/contrib`. Protocols in `scapy/layers` should be usually found
+on common networks, while protocols in `scapy/contrib` should be
+uncommon or specific.
+
+### Features
+
+Protocol-related features should be implemented within the same module
+as the protocol layers(s) (e.g., `traceroute()` is implemented in
+`scapy/layers/inet.py`).
+
+Other features may be implemented in a module (`scapy/modules`) or a
+contribution (`scapy/contrib`).
+
+### Core
+
+If you contribute to Scapy's core (e.g., `scapy/base_classes.py`,
+`scapy/packet.py`, etc.), please be very careful with performances and
+memory footprint, as it is easy to write Python code that wastes
+memory or CPU cycles.
+
+As an example, Packet().__init__() is called each time a **layer** is
+parsed from a string (during a network capture or a PCAP file
+read). Adding inefficient code here will have a disastrous effect on
+Scapy's performances.
+
+### Code review
+
+Maintainers tend to be picky, and you might feel frustrated that your
+code (which is perfectly working in your use case) is not merged
+faster.
+
+Please don't be offended, and keep in mind that maintainers are
+concerned about code maintainability and readability, commit history
+(we use the history a lot, for example to find regressions or
+understand why certain decisions have been made), performances,
+integration in Scapy, API consistency (so that someone who knows how
+to use Scapy will know how to use your code), etc.
+
+**Thanks for reading, happy hacking!**

README.md

@@ -22,6 +22,11 @@ and
 [the quick demo: an interactive session](http://www.secdev.org/projects/scapy/demo.html)
 (some examples may be outdated).
 
+# Contributing #
+
+Want to contribute? Great! Please take a few minutes to
+[read this](CONTRIBUTING.md)!
+
 # License #
 
 Scapy is free software: you can redistribute it and/or modify it under