[doc] Add contribution guidelines

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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+Contributing to iPXE
+====================
+
+Thank you for wanting to help to improve iPXE!  Your contributions can
+help to make the software better for everyone.
+
+iPXE is [licensed][licence] under the GNU GPL and is 100% open source
+software.  There are no "premium edition" versions, no in-app
+advertisements, no hidden costs.  The version that you get to download
+and use forever for free is the definitive and only version of the
+software.
+
+There are several ways in which you can contribute:
+
+Bug reports
+-----------
+
+Good quality [bug reports][issues] are always extremely welcome.
+Please describe exactly what you are trying to achieve, and the exact
+problem that you are encountering.  Include a screenshot or a
+transcription of the output from iPXE (including the exact text of any
+error messages).
+
+If you are not sure whether or not your issue is due to a bug in iPXE,
+please consider creating a [discussion][discussions] instead of an
+issue.
+
+Bug fix pull requests
+---------------------
+
+Short and simple [pull requests][pulls] that fix a known [bug][issues]
+are always welcome.  Please ensure that your pull request allows
+maintainer edits, since there is a 99% chance that it will need to be
+rebased, reworded, or otherwise modified before being merged.
+
+Minor feature pull requests
+---------------------------
+
+A small [pull request][pulls] that adds a very minor feature has some
+chance of being merged.
+
+As a bootloader, iPXE is an unusual programming environment: code size
+is extremely small (measured in bytes or kilobytes), you absolutely
+cannot use any external libraries, heap size is very limited and you
+should expect memory allocations to fail, various operations may not
+be used in some contexts (e.g. on network fast paths), and cooperative
+multitasking is implemented using event-driven non-blocking object
+interfaces.  You will need to be aware of external interoperability
+requirements and firmware quirks dating back to the 1990s.
+
+In most cases, an [issue report][issues] describing the desired
+feature is likely to get a better result than an unsolicited pull
+request.
+
+Major feature pull requests
+---------------------------
+
+A large [pull request][pulls] that adds a substantial new feature
+(e.g. a new device driver, a new cryptographic algorithm, a new script
+command, etc) has very little chance of being merged unless an
+appropriate level of [funding][funding] is provided to cover the time
+required for several rounds of code review.
+
+Please be aware that thorough code review is generally **more work**
+than code creation.  The cost to review a major feature will therefore
+generally be more expensive than the cost of simply [funding][funding]
+the development of the feature instead.
+
+Any externally developed features are **not eligible** to be signed
+for UEFI Secure Boot, and will be automatically excluded from all
+Secure Boot builds of iPXE.
+
+Funding
+-------
+
+Small donations from individual users via [GitHub Sponsors][sponsors]
+are always welcome, and help to purchase the coffee that forms the raw
+material from which iPXE is made.
+
+Around half of the development work on iPXE is commercially funded.
+Any code that is commercially funded will be pushed to the public iPXE
+repository and made immediately available to everyone under the terms
+of the GNU GPL.  Funded development has included many widely used
+features:
+
+  * Support for 64-bit CPUs
+  * Support for UEFI firmware
+  * Support for USB NICs
+  * Support for the Intel 10GbE, 40GbE, and 100GbE NICs
+  * Support for [iSCSI and other SAN booting methods][sanboot]
+  * Support for [TLS and HTTPS][crypto]
+  * Support for [AWS EC2][ec2] and [Google Cloud][gce] public clouds
+
+As noted above, funding the development of a major feature is
+generally cheaper than developing it yourself and then funding the
+code review to get it fixed up and merged.
+
+Any funded development work is automatically eligible to be signed for
+UEFI Secure Boot.
+
+To discuss funding an iPXE development, please send an email to
+<vendor-support@ipxe.org>, or create an [issue][issues] and mention
+that you would be interested in providing funding.
+
+
+[crypto]: https://ipxe.org/crypto
+[discussions]: https://github.com/ipxe/ipxe/discussions
+[ec2]: https://ipxe.org/howto/ec2
+[funding]: #funding
+[gce]: https://ipxe.org/howto/gce
+[issues]: https://github.com/ipxe/ipxe/issues
+[licence]: COPYING
+[pulls]: https://github.com/ipxe/ipxe/pulls
+[sanboot]: https://ipxe.org/cmd/sanboot
+[sponsors]: https://github.com/sponsors/ipxe