More contribution ideas

CONTRIBUTING.md

@@ -1,10 +1,9 @@
 # Contributing to cargo-mutants
 
-Pull requests are welcome.
-
-If the change is not obvious please feel free to open a bug or Github discussion first.
-
-If you are interested in working on a bug or feature please say so on the bug first to avoid wasted work, and feel free to talk or ask about the approach.
+If you're interested in adding a feature or fixing a bug, thank you! Please
+start by reading this document and opening a Github discussion or bug about the
+thing you want to do, to avoid wasted work. void wasted work, and feel free to
+talk or ask about the approach.
 
 Please also read the [DESIGN.md](DESIGN.md) file for technical information not specifically about posting contributions.
 
@@ -13,6 +12,14 @@ Please also read the [DESIGN.md](DESIGN.md) file for technical information not s
 This project is conducted in accord with the [Rust Code of
 Conduct](https://www.rust-lang.org/policies/code-of-conduct).
 
+## Try it on a new tree
+
+One of the most helpful things you can do is to try cargo-mutants on a new tree: either your own project
+or an important open source project:
+
+* Did cargo-mutants hang, error, or otherwise fail to run? If so a bug with reproduction instructions would be very helpful.
+* Were the mutants interesting or helpful in understanding coverage or test quality? Did it generate any new tests that were accepted into the tree?
+
 ## Rust Style
 
 Generally, variables and parameters should be the `snake_case` version of their
@@ -66,3 +73,12 @@ As a result, attaching a debugger to the test binary will let you see the code t
 Probably the easiest path is to just make note of the command run by the test, and then run that command yourself, under a debugger, outside of the test suite. For example, `./target/debug/cargo-mutants -d ./testdata/tree/factorial --list`.
 
 You may wish to turn off the timeouts with `-t 0`.
+
+## Generating new mutations
+
+The largest area for new work at the moment is in generating new mutations. Most of the code for this is in `visit.rs`.
+
+If you look in `mutants.out/debug.log` you can see messages like `Return type is not recognized, trying Default`.
+These might be good places to add a new more specific pattern.
+
+Also `mutants.out/unviable.txt` might suggest ways to generate new patterns that are viable.