add docs: manipulative and locomotive skills

docs/skills.md

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+## Skills
+
+*Note*: this document might currently contain some errors. I will update it later.
+
+Skills can be divided into different categories, and are ordered by increasing difficulty (from gross skills to more fine skills). The presented skills are based on [1,2,3,4,5,6], and what I could think of.
+
+* Manipulation
+    * push and pull an object
+    * grasp an object
+    * lift an object
+    * pass an object from one hand to another.
+    * strike an object (swinging a bat or stick)
+    * throw an object (control direction and force progressively)
+    * catch an object (follow flight of object, try to touch an object that has been thrown, catch an object with simple geometry,...)
+    * stacking objects (stack cubes --> stack coins)
+    * pencil tracking and handwritting
+    * dribbling (with two hands, one hand, while walking,...)
+    * put a piece on a chessboard (without moving the other pieces), placing some dominoes
+    * dual motor skills (balance cards)
+    * fold clothes
+
+* Locomotion
+    * crawl
+    * stand up
+    * balance (on two legs, on 1 leg)
+    * jump (on two legs using a support with the hands, two legs, one leg, using a rope)
+    * static locomotion
+    * climb stairs
+    * kick or roll (stationary ball, control direction and force progressively, dynamic ball, etc)
+    * running
+    * dance / gymnastic
+
+
+References:
+    [1] "Childhood development of fine motor skills" (Wikipedia): https://en.wikipedia.org/wiki/Childhood_development_of_fine_motor_skills
+    [2] "Fine Motor Development and Handwriting": http://www.therapycenterforchildren.com/child-fine-motor-development-handwriting-suffolk-county.php
+    [3] "40 Fine Motor Skills Activities": https://theimaginationtree.com/40-fine-motor-skills-activities-for-kids/
+    [4] "Early Childhood Physical Development: Gross And Fine Motor Development": https://www.mentalhelp.net/articles/early-childhood-physical-development-gross-and-fine-motor-development/
+    [5] "Gross Motor Skills: Birth to 5 years": https://www.chrichmond.org/Resource-Library/Gross-Motor-Skills-Birth-to-5-years.htm
+    [6] "Gross Motor Skills": https://www.sciencedirect.com/topics/medicine-and-dentistry/gross-motor-skill