Add User Guide and change user picture

Update README.md

Refactor code and add more error messages

Merge branch 'branch-BCD-extension'

Merge pull request #2 from justincred/branch-A-CodeQuality

Refine code quality

Add assertions

Add assertions to confirm assumptions in several places
*In the various add task methods in tasklist
to confirm the correct number of arguments

Add assertions

Add assertions to confirm assumptions in several places
*In the various add task methods in tasklist
to confirm the correct number of arguments

Merge branch 'branch-A-Level-10'

Merge branch 'branch-A-CheckStyle'

Fix merge conflict