Added the fmt.Errorf function, and how to create error constants

Errors.md

@@ -10,6 +10,14 @@ if failure {
 }
 ```
 
+or by using `fmt.Errorf`:
+
+```go
+if failure {
+	return fmt.Errorf("inverse tachyon pulse failed")
+}
+```
+
 Error strings should not start with a capital letter because they'll often be prefixed before printing:
 
 ```go
@@ -35,6 +43,14 @@ func (p ParseError) Error() string {
 }
 ```
 
+If you want to create a constant string error, you can use a named type string:
+
+```go
+type errorConst string
+
+const ErrTooManyErrors errorConst = "too many errors found."
+```
+
 Calling code would test for a special type of `error` by using a type switch:
 
 ```go