Better document the crate policy

CRATE_POLICY.md

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+# Playground crate inclusion policy
+
+The playground selects a number of root crates to include:
+
+- The top 100 crates based on [all time downloads][]
+- Crates from the [Rust cookbook][]
+
+The latest stable version of these crates are available, as well as
+whatever dependencies these crates require.
+
+## Why is there a policy?
+
+The number of crates must be restricted because time and space are
+limited resources and the playground is a volunteer-supported open source
+project. It would be infeasible to provide every possible crate.
+
+This inclusion policy is used to avoid "playing favorites" for which
+crates are available. Hand-picking crates will lead to resentment
+about which crates were not included between the playground
+maintainers and crate authors or even the broader Rust community. 
+Neither of these outcomes is desired.
+
+## Exclusion policy
+
+Occasionally, some crates that would otherwise meet the above criteria
+will not be available on the playground. A non-exhaustive list of
+reasons is:
+
+- Does not compile on Linux
+- Does not compile on the stable release channel
+- Does not compile due to invalid feature flag selection
+
+In these cases, we will temporarily exclude the crate to allow the
+playground to continue to be updated. We usually also notify the crate
+maintainers so they can adjust their crates and be re-included.
+
+## I don't like the current system!
+
+We are open to well-reasoned [alternate algorithms][], but be aware
+that any proposal would likely be expected to also provide the
+majority of implementation work.
+
+[all time downloads]: https://crates.io/crates?sort=downloads
+[Rust cookbook]: https://rust-lang-nursery.github.io/rust-cookbook/
+[alternate algorithms]: https://github.com/integer32llc/rust-playground/issues/101