docs: Add a Slicers.md document

Add some information on configuring a slicer for use with Klipper.

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

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+This document provides some tips for configuring a "slicer"
+application for use with Klipper. Common slicers used with Klipper are
+Slic3r, Cura, Simplify3D, etc.
+
+# Set the G-Code flavor to Marlin
+
+Many slicers have an option to configure the "G-Code flavor". The
+default is frequently "Marlin" and that works well with Klipper. The
+"Smoothieware" setting also works well with Klipper.
+
+# Klipper gcode_macro
+
+Slicers will often allow one to configure "Start G-Code" and "End
+G-Code" sequences. It is often convenient to define custom macros in
+the Klipper config file instead - such as: `[gcode_macro START_PRINT]`
+and `[gcode_macro END_PRINT]`. Then one can just run START_PRINT and
+END_PRINT in the slicer's configuration. Defining these actions in the
+Klipper configuration may make it easier to tweak the printer's start
+and end steps as changes do not require re-slicing.
+
+See the [example-extras.cfg](../config/example-extras.cfg) file for
+details on defining a gcode_macro.
+
+# Large retraction settings may require tuning Klipper
+
+The maximum speed and acceleration of retraction moves are controlled
+in Klipper by the `max_extrude_only_velocity` and
+`max_extrude_only_accel` config settings. These settings have a
+default value that should work well on many printers. However, if one
+has configured a large retraction in the slicer (eg, 5mm or greater)
+then one may find they limit the desired speed of retractions.
+
+If using a large retraction, consider tuning Klipper's
+[pressure advance](Pressure_Advance.md) instead. Otherwise, if one
+finds the toolhead seems to "pause" during retraction and priming,
+then consider explicitly defining `max_extrude_only_velocity` and
+`max_extrude_only_accel` in the Klipper config file.
+
+# Do not enable "coasting"
+
+The "coasting" feature is likely to result in poor quality prints with
+Klipper. Consider using Klipper's
+[pressure advance](Pressure_Advance.md) instead.
+
+Specifically, if the slicer dramatically changes the extrusion rate
+between moves then Klipper will perform deceleration and acceleration
+between moves. This is likely to make blobbing worse, not better.
+
+In contrast, it is okay (and often helpful) to use a slicer's
+"retract" setting, "wipe" setting, and/or "wipe on retract" setting.
+
+# Disable any "advanced extruder pressure" settings
+
+Some slicers advertise an "advanced extruder pressure" capability. It
+is recommended to keep these options disabled when using Klipper as
+they are likely to result in poor quality prints. Consider using
+Klipper's [pressure advance](Pressure_Advance.md) instead.
+
+Specifically, these slicer settings can instruct the firmware to make
+wild changes to the extrusion rate in the hope that the firmware will
+approximate those requests and the printer will roughly obtain a
+desirable extruder pressure. Klipper, however, utilizes precise
+kinematic calculations and timing. When Klipper is commanded to make
+significant changes to the extrusion rate it will plan out the
+corresponding changes to velocity, acceleration, and extruder
+movement - which is not the slicer's intent. The slicer may even
+command excessive extrusion rates to the point that it triggers
+Klipper's maximum extrusion cross-section check.
+
+In contrast, it is okay (and often helpful) to use a slicer's
+"retract" setting, "wipe" setting, and/or "wipe on retract" setting.