Add contributer notes

Author: LaurenzV

CONTRIBUTING.md

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+A couple of notes for people interested in contributing
+
+# Adding new features
+There are many PDF features which have only been introduced at a specific version. In addition to that, some PDF
+features are also restricted to specific sub-standards of PDF, like PDF/UA. Because of this, we need to be careful
+when adding such new features that they are no in violation of that. In particular, in case a feature is only available
+for a specific version, you need to add a check in the code so that it's only used when the current version we are exporting
+to supports it.
+
+In `crates/krilla/src/validation`, you can find some documentation on each of the validated export modes, as well as
+all the requirements needed to fulfill it, and how Typst fulfills them. So if you don't have access to the original
+specification documents, you should check those files to see whether they appear in it, and if so add appropriate checks
+to make sure we do not produce documents violating some of the standards.
+
+# Testing
+As mentioned in the README, krilla has two kinds of tests: snapshot tests and visual regression tests.
+You can create such tests by annotating your tests with the corresponding attributes (see the existing
+tests for some examples). The macros are currently somewhat undocumented and not very cleanly implemented, 
+but you should be able to find everything you need by looking at examples of existing tests.
+
+By default, if you run `cargo test`, only the snapshot tests will be run, all visual regression tests will
+be skipped. This should hopefully be enough for most cases, as running the visual regression tests requires
+a very specific setup on your computer. 
+
+If you really do need to run the visual regression tests, you can run
+`VISREG="" cargo test`. `krilla` uses my own small library called `sitro` (https://github.com/LaurenzV/sitro) that
+I built for this purpose to render PDFs, which basically is an abstraction over different PDF viewers and allows
+rendering PDF to bitmap images. However, the prerequisite is that you have the programs installed on your system and
+set the corresponding environment variables. You can take a look at the `ci.yml` file to see what the basic necessary
+setup is. On MacOS systems, you need to build another additional program that uses Quartz to render PDFs. See the README
+of `sitro` for more information on how to do that. As you can imagine, setting this up is quite a pain, but hopefully,
+for most cases it should be enough to just run the snapshot tests.
+
+In addition to that, you can prepend `SKIP_SVG` to skip the SVG-related visual regression tests. You can prepent
+`REPLACE` to overwrite the currently existing references images/snapshots, and you can use `STORE` to store the
+final PDF version of each tests in the `store` directory, which makes it easier to manually inspect them.