Add hacking md

Cargo.toml

@@ -23,6 +23,7 @@ proc_macro2_diagnostic = "0.3.0"
 quote = "1.0.45"
 
 [dev-dependencies]
+log = "0.4.29"
 trybuild = "1.0.116"
 
 [build-dependencies]

README.md

@@ -20,6 +20,7 @@
 - must have _at least one_ generic type
 - the _first_ generic type must be the `Output` type (produced when not short-circuiting)
 - the output variant (does not short-circuit) must be the _first_ variant and store the output type as the _only unnamed_ field
+- no other variant can store the Output type (TODO #72 add a nice error message)
 
 See the [full documentation](https://docs.rs/try_v2/latest/try_v2/) for specifics on the generated code.