Add E0102 error explanation

Author: GuillaumeGomez

src/librustc_typeck/diagnostics.rs

@@ -1118,6 +1118,36 @@ fn main() {
 ```
 "##,
 
+E0102: r##"
+You hit this error because the compiler the compiler lacks information
+to determine a type for this variable. Erroneous code example:
+
+```
+fn main() {
+    let x: &_; // error: cannot determine a type for this local variable
+}
+```
+
+You have two possibilities to solve this situation:
+ * Give an explicit definition of the variable
+ * Infer the variable
+
+Examples:
+
+```
+fn some_func(x: u32) {
+    // some code
+}
+
+fn main() {
+    let x = 0u32; // ok!
+    // or:
+    let x = 0;
+    some_func(x);
+}
+```
+"##,
+
 E0106: r##"
 This error indicates that a lifetime is missing from a type. If it is an error
 inside a function signature, the problem may be with failing to adhere to the
@@ -2303,7 +2333,6 @@ register_diagnostics! {
     E0085,
     E0086,
     E0090,
-    E0102,
     E0103,
     E0104,
     E0118,