Add back the E0020 error code long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes.rs

@@ -25,6 +25,7 @@ E0016: include_str!("./error_codes/E0016.md"),
 E0017: include_str!("./error_codes/E0017.md"),
 E0018: include_str!("./error_codes/E0018.md"),
 E0019: include_str!("./error_codes/E0019.md"),
+E0020: include_str!("./error_codes/E0020.md"),
 E0022: include_str!("./error_codes/E0022.md"),
 E0023: include_str!("./error_codes/E0023.md"),
 E0025: include_str!("./error_codes/E0025.md"),

src/librustc_error_codes/error_codes/E0020.md

@@ -0,0 +1,12 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+This error indicates that an attempt was made to divide by zero (or take the
+remainder of a zero divisor) in a static or constant expression. Erroneous
+code example:
+
+```compile_fail
+#[deny(const_err)]
+
+const X: i32 = 42 / 0;
+// error: attempt to divide by zero in a constant expression
+```