fix(countcharacter): added count character

- exercise was wrongly removed from master
- added again

subjects/count-character/README.md

@@ -0,0 +1,45 @@
+## count-character
+
+### Instructions
+write a function that takes a string and a character as arguments and returns the number of times the character appears in the string.
+- if the character is not in the string return 0
+- if the string is empty return 0
+
+### Expected Function
+
+```go
+func CountChar(str string, c rune) int {
+    ...
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import (   
+    "fmt"
+    "piscine"
+)
+
+func main() {
+    fmt.Println(piscine.CountChar("Hello World", 'l'))
+    fmt.Println(piscine.CountChar("5  balloons",5))
+    fmt.Println(piscine.CountChar("   ", ' '))
+    fmt.Println(piscine.CountChar("The 7 deadly sins", '7'))
+}
+```
+
+And its output :
+
+```console
+$ go run .
+3
+0
+1
+1
+```
+