doc: add lab3 requirements

lab3/README.md

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+# Lab3
+
+## Introduction
+
+In this lab, you will write unit tests for functions implemented in `main.js`. You can learn how to use classes and functions in it by uncommenting the code in it. (But remember don't commit them on GitHub)
+
+## Preparation (Important!!!)
+
+1. Sync fork on GitHub
+2. `git checkout -b lab3` (**NOT** your student ID !!!)
+
+## Requirement
+
+1. (40%) Write test cases in `main_test.js` and achieve 100% code coverage.
+2. (30%) For each function, parameterize their testcases to test the error-results.
+3. (30%) For each function, use at least 3 parameterized testcases to test the non-error-results.
+
+You can run `validate.sh` in your local to test if you satisfy the requirements.
+
+Please note that you must not alter files other than `main_test.js`. You will get 0 points if
+
+1. you modify other files to achieve requirements.
+2. you can't pass all CI on your PR.
+
+## Submission
+
+You need to open a pull request to your branch (e.g. 311XXXXXX, your student number) and contain the code that satisfies the abovementioned requirements.
+
+Moreover, please submit the URL of your PR to E3. Your submission will only be accepted when you present at both places.