Change installation instruction for delight

[hangqianjun]

Bug report
Change the current instruction for installing delight from pip; delight needs to be installed from source.

Before submitting
Please check the following:

• I have described the situation in which the bug arose, including what code was executed, information about my environment, and any applicable data others will need to reproduce the problem.
• I have included available evidence of the unexpected behavior (including error messages, screenshots, and/or plots) as well as a descriprion of what I expected instead.
• If I have a solution in mind, I have provided an explanation and/or pseudocode and/or task list.