docs: Add initial Pressure Advance tuning document

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

docs/Pressure_Advance.md

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+This document provides information on tuning the "pressure advance"
+configuration variables for a particular nozzle and filament. The
+pressure advance feature can be helpful in reducing ooze. For more
+information on how pressure advance is implemented see the
+[kinematics](Kinematics.md) document.
+
+Tuning pressure advance
+=======================
+
+Pressure advance does two useful things - it reduces ooze during
+non-extrude moves and it reduces blobbing during cornering. This guide
+uses the second feature (reducing blobbing during cornering) as a
+mechanism for measuring and tuning the pressure advance configuration.
+
+Start by changing the extruder section of the config file so that
+pressure_advance is set to 0.0. (Make sure to issue a RESTART command
+after each update to the config file so that the new configuration
+takes effect.) Then print at least 10 layers of a large hollow square
+at high speed (eg, 100mm/s). See **docs/prints/square.stl** file for
+an STL file that one may use. While the object is printing, make a
+note of which direction the head is moving during external
+perimeters. What many people see here is blobbing occurring at the
+corners - extra filament at the corner in the direction the head
+travels followed by a possible lack of filament on the side
+immediately after that corner:
+
+![corner-blob](img/corner-blob.jpg)
+
+This blobbing is the result of pressure in the extruder being released
+as a blob when the head slows down to corner.
+
+The next step is to set pressure_advance_lookahead_time to 0.0, slowly
+increase pressure_advance (eg, start with 0.05), and reprint the test
+object. (Be sure to issue RESTART between each config change.)  The
+goal is to attempt to eliminate the blobbing during cornering. (With
+pressure advance, the extruder will retract when the head slows down,
+thus countering the pressure buildup and ideally eliminate the
+blobbing.) If a test run is done with a pressure_advance setting that
+is too high, one typically sees a dimple in the corner followed by
+possible blobbing after the corner (too much filament is retracted
+during slow down and then too much filament is extruded during the
+following speed up after cornering):
+
+![corner-dimple](img/corner-dimple.jpg)
+
+The goal is to find the smallest pressure_advance value that results
+in good quality corners:
+
+![corner-good](img/corner-good.jpg)
+
+Typical pressure_advance values are between 0.05 and 0.20 (the high
+end usually only with bowden extruders).
+
+Once a good pressure_advance value is found, return
+pressure_advance_lookahead_time to its default (0.010). This parameter
+controls how far in advance to check if a head slow-down is
+immediately followed by a speed-up - it reduces pointless pressure
+changes in the head. It's possible to tune this - higher values will
+decrease the number of pressure changes in the nozzle at the expense
+of permitting more blobbing during cornering. (Tuning this value is
+unlikely to impact ooze.) The default of 10ms should work well on most
+printers.
+
+Although this tuning exercise directly improves the quality of
+corners, it's worth remembering that a good pressure advance
+configuration can reduce ooze throughout the print.
+
+Finally, once pressure_advance is tuned in Klipper, it may still be
+useful to configure a small retract value in the slicer (eg, 0.75mm)
+and to utilize the slicer's "wipe on retract option" if available.
+These slicer settings may help counteract ooze caused by filament
+cohesion (filament pulled out of the nozzle due to the stickiness of
+the plastic). It is recommended to disable the slicer's "z-lift on
+retract" option.