Update readme

Mention PWM-fan release and update extruder calibration instructions

Marlin/Version.h

@@ -48,7 +48,7 @@
    * here we define this default string as the date where the latest release
    * version was tagged.
    */
-  #define STRING_DISTRIBUTION_DATE "2019-01-16"
+  #define STRING_DISTRIBUTION_DATE "2019-01-22"
 
   /**
    * Required minimum Configuration.h and Configuration_adv.h file versions.

README.md

@@ -5,7 +5,7 @@ This is my slightly customized version of the [Marlin Firmware](https://github.c
 Feel free to discuss issues and work with me further optimizing this firmware!
 
 I am running this version on an i3 Mega Ultrabase V3 (for distinction of the different versions, check [this Thingiverse thread](https://www.thingiverse.com/groups/anycubic-i3-mega/forums/general/topic:27064)).
-Basically, this should work on every Ultrabase version that has two Z-axis endstops. The new Mega-S works too, but calibrating your extruder is mandatory since it needs to have increased E-steps to around fivefold of the original value, see the instructions below.
+Basically, this should work on every Ultrabase version that has two Z-axis endstops. The new Mega-S could work too but is not thoroughly tested. E-steps need to be set to 384 (`M92 E384.00` + `M500`), and calibration is recommended as per the instructions below.
 
 Note: This is just a firmware, not magic. A big part of print quality still depends on your slicer settings and mechanical condition of your machine. Since I have reduced the acceleration and jerk settings a bit, depending on your slicer the estimated print time might be around 20% lower. You can compensate that loss of speed by raising the general print speed without losing quality.
 
@@ -13,8 +13,9 @@ Note: This is just a firmware, not magic. A big part of print quality still depe
 
 - Special characters on any file or folders name on the SD card will cause the file menu to freeze. Simply replace or remove every special character (Chinese, Arabic, Russian, accents, German & Scandinavian umlauts, ...) from the name. Symbols like dashes or underscores are no problem.
 **Important note: On the SD card that comes with the printer there is a folder with Chinese characters in it by default. Please rename or remove it.**
-- The firmware is not reflected on the TFT-display. As the display has its own closed source firmware, you will remain to see the original Anycubic menu showing the old version number (1.1.0). If you want to find out what version really is on your printer you can send the code `M115` and read the response.
-- Possible bugs with aborting prints after pausing via display. Simply reboot the printer manually if you find yourself unable to cancel the print.
+- The firmware is not reflected on the TFT-display. As the display has its own closed source firmware, you will remain to see the original Anycubic menu showing the old version number (1.1.0). 
+- Cancelling prints via display is buggy sometimes, simply reboot the printer when the menu shows an error. Protip: Switch to OctoPrint.
+- A few parts cooling fan models (e.g. some Sunon 5015) might have trouble running slower than 100%. If that's the case, use [this release](https://github.com/davidramiro/Marlin-AI3M/releases/tag/v19.01.22-pwm).
 
 ## Why use this?
 
@@ -145,6 +146,7 @@ G26 C H200 P25 R25
 
 ### Extruder steps
 
+- Get your old E-Steps with `M503`. Look for the line starting with `M92`, the value after the `E` are your current steps.
 - Preheat the hotend with `M104 S220`
 - Send `M83` to prepare the extruder
 - Use a caliper or measuring tape and mark 120 mm (measured downwards from the extruder intake) with a pencil on the filament
@@ -153,12 +155,13 @@ G26 C H200 P25 R25
 - Measure where your pencil marking is now. If it's exactly 20 mm to the extruder, it's perfectly calibrated
 - If it's less or more than 20 mm, subtract that value from 120 mm, e.g.:
 - If you measure 25 mm, your result would be 95 mm. If you measure 15 mm, your result would be 105 mm
-- Calculate your new value: (100 mm / actually extruded filament) * 92.6
+- Calculate your new value: (100 mm / actually extruded filament) * your current E-Steps (default: 92.6)
 - For example, if your markings are at 15 mm, you'd calculate: (100/105) * 92.6 = 88.19
 - Put in the new value like this: `M92 X80.00 Y80.00 Z400.00 Exxx.xx`, replacing `x` with your value
 - Save with `M500`
 - Finish with `M82`
 
+- You can repeat the process if you want to get even more precise, you'd have to replace 92.6 with your newly calibrated value in the next calculation.
 
 ### PID tuning