Update CONTRIBUTING.md with running working version of cargo-insta (

#632)

I realize this was quite helpful for me, took me a while to figure it
out.

Also some general spring-cleaning

CONTRIBUTING.md

@@ -16,30 +16,40 @@ solve with the feature and maybe provide some ideas for how to go about that.
 
 ## Running the Tests
 
-When you want to contribute directly please make sure to run the tests and
-format the code before making a pull request. Tests are also run in CI but
-it's typically easier to run them locally.
-
 To run all tests a makefile is provided
 
 ```sh
 make test
 ```
 
-If you want to format the code you can quickly run this command:
+To format the code, run:
 
 ```sh
 make format
 ```
 
-## Conduct
+To run the version of `cargo-insta` in the working directory, run:
 
-This issue tracker follows the [Rust Code of Conduct]. For escalation or moderation
-issues please contact Armin ([email protected]) instead of the Rust moderation team.
+```sh
+cargo run -p cargo-insta -- test # (or `review` or whatever command you want to run)
+```
 
-[rust code of conduct]: https://www.rust-lang.org/policies/code-of-conduct
+...in contrast to running `cargo insta`, which invokes the installed version of
+`cargo-insta`, and so make iterating more difficult.
+
+## Writing tests
+
+If making non-trivial changes to `cargo-insta`, please add an integration test to
+`cargo-insta/tests/main.rs`. Feel free an issue if anything is unclear.
 
 ## Website / Documentation
 
-If you want to do changes to the website or documentation have a look at the
+To make changes to the website or documentation, have a look at the
 separate [insta-website](https://github.com/mitsuhiko/insta-website) repository.
+
+## Conduct
+
+This issue tracker follows the [Rust Code of Conduct]. For escalation or moderation
+issues please contact Armin ([email protected]) instead of the Rust moderation team.
+
+[rust code of conduct]: https://www.rust-lang.org/policies/code-of-conduct