add meta guide

Guide.md

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+# Adding new note
+Before committing a new note make sure the following apply:
+- Note has frontmatter according to schema and content
+- Check for any notes referencing your note and add a link there
+- Add links to other relevant notes if you reference their content
+
+# Changing Schema
+If you propose a schema change you are responsible for updating notes affected by it. All notes should pass the schema validation.
+Additionally update the `Meta.md` note.
+
+# Validation
+To ensure a valid repository structure, check this list before committing.
+
+## Dead Links
+Use [foam](https://github.com/foambubble/foam) or a similiar tool to search for and fix broken inter-note links. Links pointing to nowhere will show up as `Placeholders` if you are using foam.
+
+## Update Revision
+Don't forget to update the `rev` tag when changing a note. Update this tag to the date you last modified the note.
+
+## Formatting
+Make sure notes are correctly formatted. You can use VSCode's builtin `Format Document` function for this.
+
+## Schema Validation
+Use [mdlint](https://git.hydrar.de/mdtools/mdlint) and [mdq](https://git.hydrar.de/mdtools/mdq) to validate the schema of the notes frontmatter and make sure there are no errors:
+```shell
+mdq -c file.path --noheader knowledge | while IFS= read -r md; do
+    mdlint --ignore-missing-frontmatter "knowledge/schema.json" "$md"
+done
+```