Add mentoring notes for Java/Pangram

tracks/java/exercises/pangram/pangram/mentoring.md

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+# Mentoring
+
+## Reasonable solutions
+
+Try and guide the students towards a for-loop based solution as this requires no imports and thus can be solved with the knowledge they have so far.
+
+```java
+public class PangramChecker {
+    public boolean isPangram(String input) {
+        boolean[] present = new boolean[26];
+
+        // Loop over the alphabet and mark if letter is present
+        for (char c : input.toLowerCase().toCharArray()) {
+            if ('a' <= c && c <= 'z') {
+                present[c - 'a'] = true;
+            }
+        }
+
+        // Loop over the alphabet once more and check if a character
+        // haven't been found and return false if a letter is missing
+        for (boolean isPresent : present) {
+            if (!isPresent) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}
+```
+
+Alternatively try to guide the students towards a solutions based on set theory as this makes for a simpler solution, but it requires knowledge about Java collections.
+```java
+import java.util.HashSet;
+import java.util.Set;
+
+public class PangramChecker {
+    public boolean isPangram(String input) {
+        Set<Character> set = new HashSet<>();
+        for(char c : input.toLowerCase().toCharArray())
+            set.add(c);
+
+        Set<Character> alpha = new HashSet<>();
+        for(char c : "abcdefghijklmnopqrstuvwxyz".toCharArray())
+            alpha.add(c);
+
+        // Check if the alphabet is a subset of the input
+        return set.containsAll(alpha);
+    }
+}
+```
+
+## Common suggestions
+
+- Suggest using StringBuilder instead of String concatenating when the students are concatenating within a for-loop
+
+## Talking points
+
+- Shortcurcuit elvalutiaon, so you don't have to check the entire alphabet twice
+
+## Changelog