Add TAP.rst and USAGE.rst

Signed-off-by: Vladimir Diaz <vladimir.v.diaz@gmail.com>

TAP.rst

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+What is a TAP?
+--------------
+
+A TAP (TUF Augmentation Proposal) is a design document providing information to
+the TUF community, or describing a new feature for TUF or its processes or
+environment.  We intend TAPs to be the primary mechanisms for proposing major
+new features, for collecting community input on an issue, and for documenting
+the design decisions that have gone into TUF.
+
+Please visit the `TAPs GitHub repo <https://github.com/theupdateframework/taps>`_
+to review design changes that have been proposed to date, or to submit
+your own new feature.

USAGE.rst

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+Using TUF
+---------
+
+TUF has four major classes of users: clients, for whom TUF is largely
+transparent; mirrors, who will (in most cases) have nothing at all to do
+with TUF; upstream servers, who will largely be responsible for care and
+feeding of repositories; and integrators, who do the work of putting TUF
+into existing projects.
+
+An integration requires importing a single module into the new or existing
+software updater and calling particular methods to perform updates.  Generating
+metadata files stored on upstream servers can be handled by repository tools that
+we provide for this purpose.
+
+
+- `Integrating with a Software Updater <https://github.com/theupdateframework/tuf/tree/develop/tuf/client/README.md>`_
+
+- `Creating a TUF Repository  <https://github.com/theupdateframework/tuf/tree/develop/tuf/README.md>`_