docs(countAlpha): add subject

subjects/countalpha/README.md

@@ -0,0 +1,39 @@
+## countalpha
+
+### Instructions
+
+Write a function `CounAlpha` that takes a string in parameter and returns the number of alphabetic characters in the string.
+
+### Expected functions
+
+```go
+func CountAlpha(s string) int {
+}
+```
+
+### Usage
+
+Here is a program to test your function.
+
+```go
+package main
+
+import (
+    "fmt"
+)
+
+func main() {
+    fmt.Println(CountAlpha("Hello world"))
+    fmt.Println(CountAlpha("H e l l o"))
+    fmt.Println(CountAlpha("H1e2l3l4o"))
+}
+```
+
+And its output :
+
+```console
+$ go run .
+10
+5
+5
+```