Added Mostafa to the list of guests.

[mostafasolaiman]

Elizabeth and Philip are delighted that you are planning on coming to their wedding!
Before submitting this PR, please make sure:

• Your name is added to the list of guests with proper spelling.
• You have added the correct number of guests that are coming with and including you.
• The format of the markdown table in the guests.md file is valid.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[mostafasolaiman]

I followed all the instructions, and everything is fine.