Merge pull request #9500 from FreeJaus/bugfix-1.1.x-i3PB-GT2560_REV_A_PLUS

[1.1.x] Configs for Prusa i3 Pro B

Marlin/example_configurations/Geeetech/Prusa i3 Pro B/README.md

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+These are example configurations for the low-cost [Acrylic Prusa I3 pro B 3D Printer DIY kit](http://www.geeetech.com/acrylic-geeetech-prusa-i3-pro-b-3d-printer-diy-kit-p-917.html) and the [3DTouch auto bed leveling sensor](http://www.geeetech.com/geeetech-3dtouch-auto-bed-leveling-sensor-for-3d-printer-p-1010.html) based on:
+
+- `../GT2560/`
+- [Marlin 1.1.4 With 3DTouch / BLTouch for i3 Pro B](https://www.geeetech.com/forum/viewtopic.php?t=19846)
+
+The main characteristics of these configurations are:
+
+- The defined motherboard is `BOARD_GT2560_REV_A_PLUS`.
+- Travel limits are adjusted to the printer bed size and position.
+- An example `SKEW_CORRECTION` for a particular printer is enabled. See comments below about how to adjust it to a particular printer.
+- Using the LCD controller for bed leveling is enabled.
+  - `PROBE_MANUALLY` is enabled, which *provides a means to do "Auto" Bed Leveling without a probe*.
+  - The `LEVEL_BED_CORNERS` option for manual bed adjustment is enabled.
+  - Bilinear bed leveling is enabled, the boundaries for probing are adjusted to the glass size, and extrapolation is enabled.
+- `PRINTCOUNTER` is enabled, in order to track statistical data.
+- `INDIVIDUAL_AXIS_HOMING_MENU` is enabled, which adds individual axis homing items (Home X, Home Y, and Home Z) to the LCD menu.
+- The speaker is enabled for the UI feedback.
+- `bltouch` variant:
+  - `USE_ZMAX_PLUG` is enabled. See comments about connections below.
+  - Heaters and fans are turned off when probing.
+  - Multiple probing is set to 3.
+
+# First-time configuration
+
+## Skew factor
+
+The skew factor must be adjusted for each printer:
+
+- First, uncomment `#define XY_SKEW_FACTOR 0.0`, compile and upload the firmware.
+- Then, print [YACS (Yet Another Calibration Square)](https://www.thingiverse.com/thing:2563185). Hint, scale it considering a margin for brim (if used). The larger, the better to make error measurements.
+- Measure the printed part according to the comments in the example configuration file, and set `XY_DIAG_AC`, `XY_DIAG_BD` and `Y_SIDE_AD`.
+- Last, comment `#define XY_SKEW_FACTOR 0.0` again, compile and upload.
+
+## 3DTouch auto leveling sensor
+
+- Print a suitable mount to attach the sensor to the printer. The example configuration file is adjusted to http://www.geeetech.com/wiki/images/6/61/3DTouch_auto_leveling_sensor-1.zip
+- Unlike suggested in [geeetech.com/wiki/index.php/3DTouch_Auto_Leveling_Sensor](https://www.geeetech.com/wiki/index.php/3DTouch_Auto_Leveling_Sensor), the existing end stop switch is expected to be kept connected to Z_MIN. So, the sensor is to be connected to Z_MAX, according to Marlin's default settings. Furthermore, GT2560-A+ provides a connector for the servo next to thermistor connectors (see [GT2560](https://www.geeetech.com/wiki/images/thumb/4/45/GT2560_wiring.jpg/700px-GT2560_wiring.jpg) and [GT2560-A+](http://i.imgur.com/E0t34VU.png)).
+- Be careful to respect the polarity of the sensor when connecting it to the GT2560-A+. Unlike end stops, reversing the connection will prevent the sensor from working properly.
+- [Test](http://www.geeetech.com/wiki/index.php/3DTouch_Auto_Leveling_Sensor#Testing) and [calibrate](https://www.geeetech.com/wiki/index.php/3DTouch_Auto_Leveling_Sensor#Calibration) the sensor.