Update README.md

subjects/diamon/README.md

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 ## 🌟 Diamon
 
-diamon (pyramid + reversed pyramid)
+### Instructions
+
+Create a `diamon` function that takes a string and a number as parameters
+and return a diamon shape constructed by 2 arguments:
+ - the characters (as a string) to be repeated
+ - the size of a side (as a number)
+
+Just like pyramid + a reversed pyramid
+
+### Output example
+
+- `*` character and side of 5 :
+
+```
+    *
+   ***
+  *****
+ *******
+*********
+ *******
+  *****
+   ***
+    *
+```
+
+> No new line in last line
+
+### Notions
+
+- [nan-academy.github.io/js-training/examples/loops](https://nan-academy.github.io/js-training/examples/loops.js)
+- [nan-academy.github.io/js-training/examples/recursion](https://nan-academy.github.io/js-training/examples/recursion.js)