feat(piscine-go): add README.md for new go exercise descendcomb

subjects/descendcomb/README.md

@@ -0,0 +1,36 @@
+## descendcomb
+
+### Instructions
+
+Write a function that prints in descending order and on a single line: all possible combinations of two different two-digit numbers.
+
+These combinations are separated by a comma and a space.
+
+### Expected function
+
+```go
+func DescendComb() {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import "piscine"
+
+func main() {
+	piscine.DescendComb()
+}
+```
+
+This is the incomplete output:
+
+```console
+$ go run . | cat -e
+99 98, 99 97, 99 96, 99 95, 99 94, ..., 03 01, 03 00, 02 01, 02 00, 01 00$
+```