docs: basic description of rules

docs/rules.md

@@ -0,0 +1,50 @@
+# Rules
+
+Rules apply a schema fragment to a set of nodes selected from the original data.
+
+- [Rules](#rules)
+  - [Metadata](#metadata)
+    - [Name](#name)
+    - [Desc](#desc)
+    - [Level](#level)
+    - [Tags](#tags)
+  - [Data](#data)
+    - [Select](#select)
+    - [Filter](#filter)
+    - [Check](#check)
+
+## Metadata
+
+### Name
+
+The rule name for running single rules.
+
+### Desc
+
+The rule description for error messaging.
+
+### Level
+
+The rule log level.
+
+### Tags
+
+A list of tags for the rule. Used for inclusion and exclusion.
+
+## Data
+
+### Select
+
+JSON path used to select nodes from the data.
+
+### Filter
+
+Schema fragment used to filter selected nodes.
+
+If a node matches the `select` path but does not match this schema, it will be skipped.
+
+### Check
+
+Schema fragment used to check selected nodes.
+
+This is the real rule.