Add RESOLVER_NOTES.md

RESOLVER_NOTES.md

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+# Dino resolver
+
+Package managers use what is known as a **solver/package resolver** to identify exact versions of dependencies that are compatible with each other. Each solver has different properties and _may_ have different solutions. This is what makes things difficult: either we manually port npm's package solver to Rust, which takes a lot of time, or use a different solver such as Pubgrub (more on that later)
+
+## deno_npm
+This is the registry client and dependency resolver used by [Deno.](https://deno.com)
+
+### Pros
+- Deno has been battle-tested, and deno_npm has plenty of tests.
+- Written in Rust, no need to call out to Node to solve packages.
+
+### Cons
+- Deeply integrated with a lot of the deno_* crates, which removes the point (this is a project for learning after all!)
+- Weird Deno-related quirks, such as a [hardcoded version of @types/node](https://github.com/denoland/deno_npm/blob/8e6a3bd35249868c7266b9efca441c1bf259f0e8/src/resolution/common.rs#L152)
+
+## Pubgrub
+Used by Dart, and by extension Flutter.
+
+### Pros
+- It's fast. Like, really fast.
+- Battle-tested with Flutter and Dart, which are arguably more popular than Deno.
+- Great error messages built-in
+
+### Cons
+- The differences between the Pubgrub and the npm solver could use incompatibilities and subtle issues (such as one working fine, and the other using a broken version of a package)