docs: Explain what to do when slice is empty or there are no even elements as per review

docs: remove piscine imports and refactor to remove variable as per review

subjects/evenlength/README.md

@@ -0,0 +1,33 @@
+## evenlength
+
+### Instructions
+
+Write a function that receives a slice of strings and returns a new slice with the strings in which the length is even. When the slice is empty or there are no even strings return an empty slice.
+
+### Expected function
+
+```go
+func EvenLength(strings []string) []string {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import "fmt"
+
+func main() {
+    fmt.Println(EvenLength([]string{"Hi", "Hola", "Ola", "Ciao", "Salut", "Hallo"}))
+}
+```
+
+And its output:
+
+```console
+$ go run .
+[Hi Hola Ciao]
+```