Updated SliceTricks (markdown)

SliceTricks.md

@@ -11,6 +11,12 @@ a = append(a, b...)
 ```go
 b = make([]T, len(a))
 copy(b, a)
+
+// These two are often a little slower than the above one,
+// but they would be more efficient if there are more
+// elements to be appended to b.
+b = append([]T(nil), a...)
+b = append(a[:0:0], a...
 ```
 
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