Update free-science-books.md

[Hridoy-31]

What does this PR do?

Physics section updated

For resources

Description

Added "Lecture Notes on General Relativity" by Holland & Sanders

Checklist:

• [x] Not a duplicate
• [x] Included author(s) if appropriate
• [x] Lists are in alphabetical order
• [x] Needed indications added (PDF, access notes, under construction)

[Hridoy-31]

It will be very much helpful to me if you add the "hacktoberfest-accepted" label to this PR

[eshellman]

have added the keyword to the repo