docs: add to README

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

README.md

@@ -3,7 +3,7 @@
 I'm trying the [advent of code](https://adventofcode.com/2023) in Rust this
 year to learn Rust. I'm not trying to be fast and place on the leaderboards
 (which also require working at midnight, which I'm mostly not interested in
-doing), I'm trying to be somewhat elegant and learn new things.
+doing), I'm trying to be somewhat elegant and learn new things in Rust.
 
 ## Formatting and linting
 
@@ -44,4 +44,21 @@ feature in the nightlies (`01` supports it), but I wanted `cargo fmt` and
 approach.
 
 Features used in each vary. For example, `05` has an optional progress bar
-(opt-out). `08` uses a crate.
+(opt-out). Over time, I've been cleaning up the older problems based on what
+I've learned in newer problems, so looking at the history for a file might be
+instructive.  I started using external crates like `itertools` & `derive_more`
+around 10-12 or so, but backported a lot of the cleanups later.
+
+A few of the crates I'm using or have used:
+
+- `cached`: Python's `itertools.cache` basically
+- `derive-new`: Powerful `new` creation (supports default, unlike `derive_more`'s `Constructor`.
+- `derive_more`: Adds useful derives not part of the stdlib (like Add)
+- `grid`: A simple 2D array library
+- `indicatif`: Progress bars
+- `log`, `env_logger`, `test-log`: logging facilities
+- `num`: Needed `lcm` in a problem.
+- `rayon` (not actively used): Easy multithreading
+- `strum`: Powerful enum tools like conversion with strings & iteration over enums
+
+I added some documentation to 13 to try `cargo doc`.