Add Feature Flags page with feature matrix

Signed-off-by: George Melikov <mail@gmelikov.ru>

docs/Basic Concepts/Feature Flags.rst

@@ -0,0 +1,53 @@
+Feature Flags
+=============
+
+ZFS on-disk formats were originally versioned with a single number,
+which increased whenever the format changed. The numbered approach was
+suitable when development of ZFS was driven by a single organisation.
+
+For distributed development of OpenZFS, version numbering was
+unsuitable. Any change to the number would have required agreement,
+across all implementations, of each change to the on-disk format.
+
+OpenZFS feature flags – an alternative to traditional version numbering
+– allow **a uniquely named pool property for each change to the on-disk
+format**. This approach supports:
+
+-  format changes that are independent
+-  format changes that depend on each other.
+
+Compatibility
+-------------
+
+Where all *features* that are used by a pool are supported by multiple
+implementations of OpenZFS, the on-disk format is portable across those
+implementations.
+
+Features that are exclusive when enabled should be periodically ported
+to all distributions.
+
+Reference materials
+-------------------
+
+`ZFS Feature Flags <http://web.archive.org/web/20160419064650/http://blog.delphix.com/csiden/files/2012/01/ZFS_Feature_Flags.pdf>`_
+(Christopher Siden, 2012-01, in the Internet
+Archive Wayback Machine) in particular: "… Legacy version numbers still
+exist for pool versions 1-28 …".
+
+`zpool-features(5) man page <../man/5/zpool-features.5.html>`_ - OpenZFS
+
+`zpool-features <http://illumos.org/man/5/zpool-features>`__ (5) – illumos
+
+Feature flags implementation per OS
+-----------------------------------
+
+.. raw:: html
+
+   <div class="man_container">
+
+.. raw:: html
+   :file: ../_build/zfs_feature_matrix.html
+
+.. raw:: html
+
+   </div>