docs(book): Add usage chapter

Adds a usage chapter wich outlines how to initalize a project and update a
default test.

docs/book/src/SUMMARY.md

@@ -3,7 +3,7 @@
 
 # Quickstart
 - [Installation](./quickstart/install.md)
-- [Usage]()
+- [Usage](./quickstart/usage.md)
 - [Concepts]()
 - [Configuration]()
 

docs/book/src/quickstart/usage.md

@@ -0,0 +1,121 @@
+# Usage
+`typst-test` is a command line program, it can be run by simply invoking it in your favorite shell and passing the appropriate arguments.
+
+If you opened a shell in a folder `project` and `typst-test` is at `project/bin/typst-test`, then you can run it using `./project/bin/typst-test`.
+Placing it directly in your project is most likely not what you will do or want to do.
+You should install it to a directory which is contained in your `PATH`, allowing you to simply run it using `typst-test`.
+How to add such folders to your `PATH` depends on your operating system, but if you installed `typst-test` using one of the recommended methods in [Installation](.install.md), then such a folder should be chosen for you.
+
+<div class="warning">
+
+For the remainder of this document `tt` is used in favor of `typst-test` whenever a command line example is shown.
+When you see an example such as
+```bash
+tt run -e 'name(~id)'
+```
+it is meant to be run as
+```bash
+typst-test run -e 'name(~id)'
+```
+
+You can also define an alias of the same name to make typing it easier.
+
+</div>
+
+`typst-test` requires a certain project structure to work, if you want to start testing your project's code, you can create an example test and the required directory structure using the `init` command.
+
+```bash
+tt init
+```
+
+This will create the default example to give you a graps at where tests are located and how they are structured.
+`typs-test` will look for the project root by checking for directories containing a `typst.toml` manifest file.
+This is because `typst-test` is primarily aimed at developers of packages, if you want to use a different project root, or don't have a `typst-manifest` you can provide the root directory using the `--root` like so.
+
+```bash
+tt init --root ./path/to/root/
+```
+
+Keep in mind that you must pass this option for every command that operates on a project.
+Alternatively the `TYPST_ROOT` environment variable can be set to the project root.
+
+Further examples assume the existence of a manifest or the `TYPST_ROOT` variable being set
+If you're just following along and don't have a package to test this with, you can use a an empty project with the following manifest:
+
+```toml
+[package]
+name = "foo"
+description = "A fancy Typst package!"
+authors = ["John Doe"]
+license = "MIT"
+
+entrypoint = "src/lib.typ"
+version = "0.1.0"
+```
+
+Once the project is initialized, you can run the example test to see that everything works.
+
+```bash
+tt run example
+```
+
+You should see something along the lines of
+
+```txt
+Running tests
+        ok example
+
+Summary
+  1 / 1 passed.
+```
+
+Let's edit the test to actually do something, the default example test can be found in `<project>/tests/example/` and simply contains `Hello World`.
+Let's write something else in there and see what happens
+```diff
+-Hello World
++Typst is Great!
+```
+
+Once we run `typst-test` again we'll see that the test no longer passes:
+
+```txt
+Running tests
+    failed example
+           Page 1 had 1292 deviations
+           hint: Diff images have been saved at '<project>/tests/example/diff'
+
+Summary
+  0 / 1 passed.
+```
+
+`typst-test` has compared the reference output from the original `Hello World` docuemnt to the new document and determined that they don't match.
+It also told you where you can inspect the difference, the `<project>/test/example` contains a `diff` directory.
+You can take a look to see what changed by also looking at the `out` and `ref` directories, these contain the output of the current test and the expected reference output respectively.
+
+Well, but this wasn't a mistake, this was a deliberate change.
+So let's update the references to reflect that.
+For this we use the appropriately named `update` command:
+
+```bash
+tt update example
+```
+
+You should see output similar to
+
+```txt
+Updating tests
+   updated example
+
+Summary
+  1 / 1 updated.
+```
+
+and the test should once again pass.
+
+<div class="warning">
+
+Beware that `update` will by default update all test which aren't ignored, this may change before 0.1.0 is released.
+Ensure you always have your references checked into your VCS to avoid losing them.
+Check out the sections about test sets to learn more about how tests are selected.
+
+</div>