Update readings.md

[yth]

Adding interesting resources for algorithm and a very beginner friendly resource for category theory.

[waciumawanjohi]

The algorithms book looks like a well reviewed resource. https://www.goodreads.com/book/show/20613168-algorithms
Appropriate to add.

Category Theory isn't an undergrad CS topic. Grepping the curricular guidelines for category theory doesn't bring up one hit. I don't see an argument to add that book.

[yth]

There are already two category theory books in the extra reading list, which is why I thought it was an appropriate topic to add a resource for.

"A Taste of Category Theory for Computer Scientists" is not well-known, but its credentials are impeccable. It's by Benjamin C Pierce, the author of Type and Programming Languages. It's the precursor to his book "Basic Category Theory for Computer Scientists". Arguably, this version is even more approachable than the book. It might be less well-known because it is designated as a research report. It's possibly the best category theory resource I have found for self-study. That's why I thought it should be included here.