Added README.md file for Number of Unique Subjects Taught by Each Teacher

Author: Pratik-Khose

2495-number-of-unique-subjects-taught-by-each-teacher/README.md

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+<h2><a href="https://leetcode.com/problems/number-of-unique-subjects-taught-by-each-teacher">Number of Unique Subjects Taught by Each Teacher</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Table: <code>Teacher</code></p>
+
+<pre>
++-------------+------+
+| Column Name | Type |
++-------------+------+
+| teacher_id  | int  |
+| subject_id  | int  |
+| dept_id     | int  |
++-------------+------+
+(subject_id, dept_id) is the primary key (combinations of columns with unique values) of this table.
+Each row in this table indicates that the teacher with teacher_id teaches the subject subject_id in the department dept_id.
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Write a solution to calculate&nbsp;the number of unique subjects each teacher teaches in the university.</p>
+
+<p>Return the result table in <strong>any order</strong>.</p>
+
+<p>The&nbsp;result format is shown in the following example.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+Teacher table:
++------------+------------+---------+
+| teacher_id | subject_id | dept_id |
++------------+------------+---------+
+| 1          | 2          | 3       |
+| 1          | 2          | 4       |
+| 1          | 3          | 3       |
+| 2          | 1          | 1       |
+| 2          | 2          | 1       |
+| 2          | 3          | 1       |
+| 2          | 4          | 1       |
++------------+------------+---------+
+<strong>Output:</strong>  
++------------+-----+
+| teacher_id | cnt |
++------------+-----+
+| 1          | 2   |
+| 2          | 4   |
++------------+-----+
+<strong>Explanation:</strong> 
+Teacher 1:
+  - They teach subject 2 in departments 3 and 4.
+  - They teach subject 3 in department 3.
+Teacher 2:
+  - They teach subject 1 in department 1.
+  - They teach subject 2 in department 1.
+  - They teach subject 3 in department 1.
+  - They teach subject 4 in department 1.
+</pre>