Update README.md

subjects/findmissingnumber/README.md

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+## find-missing-number
+
+### Instructions
+
+Write a function that takes an array of numbers and returns the missing number.
+- If the array is empty, return `-1`.
+- If the array contains only one number or there is no missing number, return `-1`.
+- if the array contains more than one missing number, return the minimum missing number.
+
+### Expected function
+```go
+func FindMissingNumber(numbers []int) int {
+    // Your code here
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import "fmt"
+
+func main(){
+    fmt.Println(FindMissingNumber([]int{1, 2, 5,3, 6, 7, 8, 9, 10}))
+    fmt.Println(FindMissingNumber([]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}))
+    fmt.Println(FindMissingNumber([]int{-10,12,32})
+}
+```
+
+and the output should be:
+
+```console
+$ go run .
+4
+-1
+-9
+```