Ender3 - Firmware config + README update

creality-ender3/README.md

@@ -5,6 +5,8 @@
 3. [Print profiles](#print-profiles)
 4. [Filament set up](#filament-set-up)
 5. [Raspberry Pi 4 / Octoprint case](#raspberry-pi-4--octoprint-case)
+6. [Marlin custom firmware](#marlin-custom-firmware)
+7. [3D printer custom parts](#3d-printed-custom-parts)
 
 # Octoprint backup archive
 
@@ -169,4 +171,38 @@ Final result:
 #
 # m h  dom mon dow   command
 @reboot raspi-gpio set 4 op dh
-```
+```
+
+# Marlin custom firmware
+
+## Installed firmware
+
+- Currently used tag: [`2.1.2.4`](https://github.com/MarlinFirmware/Marlin/tree/2.1.2.4)
+- Details: [Firmware sources and custom configuration](./Marlin-2.1.2.4/) (local repo)
+- Last compilation date: `2024-10-10 @ 09:53`
+
+## Useful resources
+
+### Firmware configuration:
+- [Marlin 2.0.x Firmware Guide for BTT SKR Mini E3 v2.0 Ender 3](https://wallabag.igox.org/view/202)
+- [Marlin 2.0.8 guide - PID autotune](https://wallabag.igox.org/view/203)
+- [Marlin 2.0.8 guide - Bed Leveling - BLTouch, Inductive Sensor](https://wallabag.igox.org/view/204)
+
+### Theorical documentation:
+- [[MARLIN] Déterminer les dimensions / positions de la zone d'impression de son imprimante (cartésienne / CoreXY)](https://web.archive.org/web/20241010080558/https://www.lesimprimantes3d.fr/forum/topic/47020-marlin-déterminer-les-dimensions-positions-de-la-zone-dimpression-de-son-imprimante-cartésienne-corexy/)
+- [comprendre la zone de palpage avec un ABL afin de paramétrer son Marlin](https://www.lesimprimantes3d.fr/forum/topic/43747-quelques-éléments-pour-mieux-comprendre-la-zone-de-palpage-avec-un-abl-afin-de-paramétrer-son-marlin/)
+
+### Printer calibration:
+
+Proceed with the different calibrations in the above order:
+
+- [Marlin PID Auto-Tune - A Complete Guide](https://3dprintinggeek.com/marlin-pid-tuning/) 
+    - :warnning: First do the PID Auto-Tune for the **bed**. Else you may have false positive thermal runout.
+- [Z-Offset Wizard – A Complete Guide](https://3dprintscape.com/z-offset-wizard-complete-guide/)
+- Linear Advance:
+    - [Explanation](https://marlinfw.org/docs/features/lin_advance.html)
+    - [Pattern generation tool](https://marlinfw.org/tools/lin_advance/k-factor.html)
+
+
+# 3D printed custom parts
+