DEV-3198 add(docs): add readme

subjects/digitlen/READMED.md

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+## digit-len
+
+### Instructions 
+
+Write a function that takes two integers and returns the number of digits in the first integer by the base of the second integer.
+- The second integer must be between 2 and 36. if not, the function returns `-1`.
+- If the first integer is negative , reverse the sign and count the digits.
+
+
+### Expected function
+
+```go
+func DigitLen(n, base int) int {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function :
+
+```go
+package main
+
+import "fmt"
+
+func main(){
+    fmt.Println(DigitLen(100, 10))
+    fmt.Println(DigitLen(100, 2))
+    fmt.Println(DigitLen(-100, 16))
+    fmt.Println(DigitLen(100, -1))
+
+}
+```
+
+And its output :
+
+```console
+$ go run . | cat -e
+3
+7
+2
+-1
+```