readme: move currently unimplemented use cases to todo section

We want to implement these features soon, but they don't exist at the moment.

README.md

@@ -17,8 +17,6 @@ the `cargo-guppy` CLI as well:
 * track existing dependencies for a crate or workspace
 * query direct or transitive dependencies of a subset of packages — useful when some packages have greater assurance or
   reliability requirements
-* receive CI feedback if a dependency is added, updated or removed
-* receive CI feedback if a package goes from not being included in a high-assurance subset to being included
 * figure out what's causing a particular crate to be included as a dependency
 * iterate over reverse dependencies of a crate in topological order
 * iterate over some or all links (edges) in a dependency graph, querying if the link is a build, dev or regular
@@ -28,6 +26,8 @@ the `cargo-guppy` CLI as well:
 
 Still to come:
 
+* receive CI feedback if a dependency is added, updated or removed
+* receive CI feedback if a package goes from not being included in a high-assurance subset to being included
 * queries based on features
 * evaluation of target specs for [platform-specific dependencies](https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#platform-specific-dependencies)
 * a command-line query language