docs: add guidelines for registering new sign modes (#11056)

## Description

ref #11055



---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

proto/cosmos/tx/signing/v1beta1/signing.proto

@@ -7,6 +7,13 @@ import "google/protobuf/any.proto";
 option go_package = "github.com/cosmos/cosmos-sdk/types/tx/signing";
 
 // SignMode represents a signing mode with its own security guarantees.
+//
+// This enum should be considered a registry of all known sign modes
+// in the Cosmos ecosystem. Apps are not expected to support all known
+// sign modes. Apps that would like to support custom  sign modes are
+// encouraged to open a small PR against this file to add a new case
+// to this SignMode enum describing their sign mode so that different
+// apps have a consistent version of this enum.
 enum SignMode {
   // SIGN_MODE_UNSPECIFIED specifies an unknown signing mode and will be
   // rejected.