README: document UI changes when compared to gofmt

We've accumulated a few special behaviors since the last release.
Document them. Especially the special diagnostic comment.

README.md

@@ -10,9 +10,16 @@ replacement. Running `gofmt` after `gofumpt` should be a no-op. For example:
 
  gofumpt -l -w .
 
-Most of the Go source files in this repository belong to the Go project.
+Some of the Go source files in this repository belong to the Go project.
 The added formatting rules are in the `format` package.
 
+Beyond the [added rules below](#Added-rules), the tool differs from gofmt in the
+following ways:
+
+* Vendor directories are skipped unless given as explicit arguments
+* The added rules are not applied to generated Go files
+* The `-r` rewrite feature is removed in favor of `gofmt -r`
+
 ### Added rules
 
 No empty lines following an assignment operator
@@ -583,6 +590,21 @@ before sending a pull request.
 We also use the `#gofumpt` channel over at the
 [Gophers Slack](https://invite.slack.golangbridge.org/) to chat.
 
+When reporting a formatting bug, insert a `//gofumpt:diagnose` comment.
+The comment will be rewritten to include useful debugging information.
+For instance:
+
+```
+$ cat f.go
+package p
+
+//gofumpt:diagnose
+$ gofumpt f.go
+package p
+
+//gofumpt:diagnose v0.1.1-0.20211103104632-bdfa3b02e50a -lang=v1.16
+```
+
 ### License
 
 Note that much of the code is copied from Go's `gofmt` command. You can tell