encoding/csv: improve Reader docs (#6828)

vlib/encoding/csv/readme.md

@@ -0,0 +1,19 @@
+## Reader example
+
+```v
+import encoding.csv
+
+data := 'x,y\na,b,c\n'
+mut parser := csv.new_reader(data)
+
+// read each line
+for {
+    items := parser.read() or {break}
+    println(items)
+}
+```
+It prints:
+```
+['x', 'y']
+['a', 'b', 'c']
+```