[PATCH 3/3] mdadm: Add README.md

[Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>]

Describe supported metadata types, add step-by-step patch sending
instruction, mention minimally supported kernel version and licensing.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
---
 README.md | 83 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)
 create mode 100644 README.md

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+**mdadm** is a utility used to create and manage **software RAID** devices implemented through
+**Multiple devices driver (MD)** in kernel. It supports following RAID metadata formats:
+
+* [Linux native RAID](https://raid.wiki.kernel.org/index.php/RAID_superblock_formats):
+
+  Known as **native** or **native RAID**. First and default metadata format. Metadata management
+  is implemented in **MD driver**.
+
+* Matrix Storage Manager Support (no reference, metadata format documentation is proprietary).
+
+  Known as **IMSM**. Metadata format developed and maintained by **Intel®** as a part of **VROC**
+  solution. There are some functional differences between **native** and **imsm**. The most
+  important difference is that the metadata is managed from userspace.
+
+  **CAUTION:** **imsm** is compatible with **Intel RST**, however it is not officially supported.
+  You are using it on your own risk.
+
+* [Common RAID DDF Specification Revision](https://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf)
+
+    **IMPORTANT:** DDF is in **maintenance only** mode. There is no active development around it.
+    Please do not use it in new solutions.
+
+# How to Contribute
+
+ **mdadm** is hosted on [kernel.org](https://kernel.org/). You can access repository
+[here](https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git).
+
+It is maintained similarly to kernel, using *mailing list*. Patches must be send through email.
+Please familiarize with general kernel
+[submitting patches](https://www.kernel.org/doc/html/v4.17/process/submitting-patches.html)
+documentation. Formatting, tags and commit message guidelines applies to **mdadm**.
+
+## Sending patches step-by-step
+
+To maximize change of patches being taken, follow this instruction when submitting:
+
+1. Create possibly logically separated commits and generate patches:
+
+   Use ``git format-patch --cover-letter --signoff -v <nr>`` to create patches:
+   * ``--cover-letter`` can be skipped if it is only one patch;
+   * ``--signoff`` adds sign-off tag;
+   * ``-v <nr>`` indicates review revision number, sender should increment it before resending.
+
+2. Check style of every patch with kernel
+   [checkpatch](https://docs.kernel.org/dev-tools/checkpatch.html) script:
+
+   It is important to keep same coding style that is why in **mdadm**
+   [kernel coding style](https://www.kernel.org/doc/html/v4.10/process/coding-style.html)
+   is preferred. ``checkpath --no-tree <patch_file>`` can be used to verify patches.
+   Following checkpatch issues can be ignored:
+   - New typedefs.
+   - comparing with *True/False*.
+   - kernel *MAINTAINERS* file warning.
+   - *extern* keyword in headers.
+
+3. Send patches using ``git send-mail --to=linux-raid@vger.kernel.org <cover-letter> <patch1> <patch2> (...)``
+
+# Maintainers
+
+It is good practice to add **mdadm maintainers** to recipients for patches:
+
+- Jes Sorensen <jes@trained-monkey.org>;
+- Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>;
+
+Adding **MD maintainers** could be reasonable, especially if patches may affect MD driver:
+
+- Song Liu <song@kernel.org>;
+- Yu Kuai <yukuai3@huawei.com>;
+
+# Reviewers
+
+**mdadm** utility is not part of kernel tree, so there is no certificated *Reviewers* list. Everyone
+can comment on mailing list, last decision (and merging) belongs to maintainers.
+
+# Minimal supported kernel version
+
+We do not support kernel versions below **v3.10**. Please be aware that maintainers may remove
+workarounds and fixes for legacy issues.
+
+# License
+
+It is released under the terms of the **GNU General Public License version 2** as published
+by the **Free Software Foundation**.
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