Merge rust-bitcoin/rust-bitcoin#2207: policy: Add section on standard set of derives

cb42c74f58 policy: Add section on standard set of derives (Tobin C. Harding)
ec1a5a25c7 CONTRIBUTING: Remove stale links (Tobin C. Harding)

Pull request description:

  We can have a standard set of derives to make it easier for new devs to work out what to use and also to help us move towards a uniform set in preparation for crate stabilization.

  This is not me imposing my view but rather a place for the discussion to happen, could have been a discussions topic also? Using "open" instead of "draft" to aid visibility. Probably requires more than the usual amount of acks.

ACKs for top commit:
  apoelstra:
    ACK cb42c74f58
  Kixunil:
    ACK cb42c74f58

Tree-SHA512: b4c4094ea3652e92a5bea90e16f13971202710166524cc15abef5e8318ba5f59df084f5246331870fc641456b49d4e35d266c937375bdc5035f03699a7d4c1b9

CONTRIBUTING.md

@@ -18,8 +18,6 @@ changes to this document in a pull request.
   * [Peer review](#peer-review)
   * [Repository maintainers](#repository-maintainers)
 - [Coding conventions](#coding-conventions)
-  * [Formatting](#formatting)
-  * [MSRV](#msrv)
   * [Naming conventions](#naming-conventions)
   * [Upgrading dependencies](#upgrading-dependencies)
   * [Unsafe code](#unsafe-code)
@@ -380,6 +378,27 @@ sure, feel free to ask. If we determine panicking is more practical it must be d
 panics that could theoretically occur because of bugs in our code must not be documented.
 
 
+#### Derives
+
+We try to use standard set of derives if it makes sense:
+
+```
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+enum Foo {
+    Bar,
+    Baz,
+}
+```
+
+For types that do should not form a total or partial order, or that technically do but it does not
+make sense to compare them, we use the `Ordered` trait from the
+[`ordered`](https://crates.io/crates/ordered) crate. See `absolute::LockTime` for an example.
+
+For error types you likely want to use `#[derive(Debug, Clone, PartialEq, Eq)]`.
+
+See [Errors](#errors) section.
+
+
 #### Attributes
 
 - `#[track_caller]`: Used on functions that panic on invalid arguments