Update readme with instruction for developing.

Change DiagnosticGenerator -> Linter since this is much easier to understand.

printer-linter/README.md

@@ -8,4 +8,26 @@ From the Cura root folder.
 ```python3 printer-linter/src/terminal.py "flashforge_dreamer_nx.def.json" "flashforge_base.def.json" --fix  --format```
 
 ## Developing
+### Printer Linter Rules
+Inside ```.printer-linter``` you can find a list of rules. These are seperated into roughly three categories. 
+
+1. Checks
+   1. These rules are about checking if a file meets some requirements that can't be fixed by replacing its content. 
+   2. An example of a check is ```diagnostic-mesh-file-extension``` this checks if a mesh file extension is acceptable.
+2. Format
+   1. These rules are purely about how a file is structured, not content.
+   2. An example of a format rule is ```format-definition-bracket-newline``` This rule says that when assigning a dict value the bracket should go on a new line.
+3. Fixes
+   1. These are about the content of the file.
+   2. An example of a fix is ```diagnostic-definition-redundant-override``` This removes settings that have already been defined by a parent definition
+
+### Linters
+Linters find issues within a file. There are separate linters for each type of file. The linter that is used is decided by the create function in factory.py. All linters implement the abstract class Linter.
+
+A Linter class returns an iterator of Diagnostics, each diagnostic is an issue with the file. The diagnostics can also contain suggested fixes.
+
+### Formatters
+Formatters load a file reformat it and write it to disk. There are separate formatters for each file type. All formatters implement the abstract class Formatter.
+
+Formatters should format based on the Format rules in .printer-linter