Add error explanation for E0070

Author: GuillaumeGomez

src/librustc_typeck/diagnostics.rs

@@ -170,6 +170,30 @@ Since `return;` is just like `return ();`, there is a mismatch between the
 function's return type and the value being returned.
 "##,
 
+E0070: r##"
+You tried to change the value of a const variable, which is not possible. Bad
+example:
+
+```
+const SOME_CONST : i32 = 12;
+
+fn some_function() {
+    SOME_CONST = 14; // error !
+}
+```
+
+Constant variables' value can be changed once it has been set. Please take a
+look to static keyword if you want something similar but mutable:
+
+```
+static mut SOME_NOT_CONST : i32 = 12;
+
+fn some_function() {
+    SOME_NOT_CONST = 14; // that's good !
+}
+```
+"##,
+
 E0081: r##"
 Enum discriminants are used to differentiate enum variants stored in memory.
 This error indicates that the same value was used for two or more variants,
@@ -658,7 +682,6 @@ register_diagnostics! {
     E0060,
     E0061,
     E0068,
-    E0070,
     E0071,
     E0072,
     E0073,