Explanation about crate version upgrades.

CRATE_POLICY.md

@@ -5,8 +5,31 @@ 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.
+The latest stable version of these crates are available, as well as whatever dependencies these
+crates require. However, the selected versions of direct or transient dependencies may be **other**
+than current stable (or most recent unstable) versions, because they have to be versions specified
+by their dependent crates. (See also [a comment on issue
+#780](https://github.com/rust-lang/rust-playground/issues/780#issuecomment-1047942472) and [another
+comment on the same
+issue](https://github.com/rust-lang/rust-playground/issues/780#issuecomment-1048140676).)
+
+## Version updates
+
+- We usually upgrade all the crates at once, since we have to rebuild it from scratch if even one
+  crate changes. (See also [a comment on issue
+  #568](https://github.com/rust-lang/rust-playground/pull/568#issuecomment-577273010).)
+- As a rule of thumb, we upgrade the crates about twice a year.
+
+  Yes, some crates (especially unstable ones) may progress faster. However, regardless of how
+  frequently we upgrade the crates, there will be either breaking changes for existing playgrounds
+  (gists), or new functionality that is not available in the playground yet.
+
+  Either way, increasing upgrade frequency for its own sake doesn't fix this problem, and it would
+  only increase the amount of work for the maintainers.
+- Unless you've created a pull request about version upgrades in the past and it was accepted,
+  please don't. Manually editing `Cargo.toml` is not appropriate. For example, this change does not
+  update `crate-information.json`. (See [a comment on issue
+  #1076](https://github.com/rust-lang/rust-playground/pull/1076#issuecomment-2260701610).)
 
 ## Why is there a policy?