subject(square-root): add readmed

subjects/squareroot/README.md

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+## Square-root
+
+### Instructions
+Write a function that takes a number and returns the square root of that number.
+- If the number is less than one or the number does not have an integer square root, return `-1`.
+- The square root of a number is the number divided by two until the number is less than or equal to one.
+
+### Expected function
+```go
+func SquareRoot(number int) int {
+    // Your code here
+}
+```
+
+### Usage
+
+```go
+package main
+
+import "fmt"
+
+func main() {
+    fmt.Println(SquareRoot(9))
+    fmt.Println(SquareRoot(16))
+    fmt.Println(SquareRoot(25))
+    fmt.Println(SquareRoot(26))
+    fmt.Println(SquareRoot(0))
+}
+```
+
+and the output should be:
+
+```console
+$ go run . 
+3
+4
+5
+-1
+-1
+```