docs: Add a features document

Add a document describing some high-level features in Klipper.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

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+Klipper is an experimental 3d printer firmware. It has several
+compelling features:
+
+* High precision stepper movement. Klipper utilizes an application
+  processor (such as a low-cost Raspberry Pi) when calculating printer
+  movements. The application processor determines when to step each
+  stepper motor, it compresses those events, transmits them to the
+  micro-controller, and then the micro-controller executes each event
+  at the requested time.  Each stepper event is scheduled with a
+  precision of no less than 50 micro-seconds. The software does not
+  use kinematic estimations (such as the Bresenham algorithm) -
+  instead it calculates precise step times based on the physics of
+  acceleration and the physics of the machine kinematics. More precise
+  stepper movement translates to quieter and more stable printer
+  operation.
+
+* Best in class performance. Klipper is able to achieve high stepping
+  rates on both new and old micro-controllers. Even an old 8bit AVR
+  micro-controller can obtain rates up to 150K steps per second. On
+  more recent ARM micro-controllers, rates over 350K steps per second
+  are possible. Higher stepper rates enable higher print
+  velocities. The stepper event timing remains precise even at high
+  speeds which improves overall stability.
+
+* Configuration via simple config file. There's no need to reflash the
+  micro-controller to change a setting. All of Klipper's configuration
+  is stored in a standard config file which can be easily edited. This
+  makes it easier to setup and maintain the hardware.
+
+* Portable code. Klipper works on both ARM and AVR
+  micro-controllers. Existing "reprap" style printers can run Klipper
+  without hardware modification - just add a Raspberry Pi. Klipper's
+  internal code layout makes it easier to support other
+  micro-controller architectures as well.
+
+* Simpler code. Klipper uses a very high level language (Python) for
+  most code. The kinematics algorithms, the gcode parsing, the heating
+  and thermistor algorithms, etc. are all written in Python. This
+  makes it easier to develop new functionality.
+
+* Advanced features. Klipper implements the "pressure advance"
+  algorithm for extruders. When properly tuned, pressure advance
+  reduces extruder ooze. Klipper also implements a novel "stepper
+  phase endstop" algorithm that can dramatically improve the accuracy
+  of typical endstop switches. When properly tuned it can improve a
+  print's first layer bed adhesion.
+
+To get started with Klipper, read the [installation](Installation.md)
+guide.
+
+Common features supported by Klipper
+====================================
+
+Klipper supports many standard 3d printer features:
+
+* Works with Octoprint. This allows the printer to be controlled using
+  a regular web-browser. The same Raspberry Pi that runs Klipper can
+  also run Octoprint.
+
+* Standard G-Code support. Common g-code commands that are produced by
+  typical "slicers" are supported. One may continue to use Slic3r,
+  Cura, etc. with Klipper.
+
+* Constant speed acceleration support. All printer moves will
+  gradually accelerate from standstill to cruising speed and then
+  decelerate back to a standstill.
+
+* "Lookahead" support. The incoming stream of G-Code movement commands
+  are queued and analyzed - the acceleration between movements in a
+  similar direction will be optimized to reduce print stalls and
+  improve overall print time.
+
+* Support for both delta printers and cartesian style printers.
+
+Step Benchmarks
+===============
+
+Below are the results of stepper performance tests. The numbers shown
+represent total number of steps per second on the micro-controller.
+
+| Micro-controller  | 1 stepper active | 3 steppers active |
+| ----------------- | ---------------- | ----------------- |
+| 20Mhz AVR         | 158.7K           | 103K              |
+| 16Mhz AVR         | 126.9K           | 82K               |
+| Arduino Due (ARM) | 352.9K           | 288K              |