docs(osv-linter): Add a README.md (#304)

This commit adds a README.md to provide interested parties with some
further details and guidance about the OSV linter

Signed-off-by: Andrew Pollock <apollock@google.com>

tools/osv-linter/README.md

@@ -0,0 +1,103 @@
+# OSV Record Linter
+
+A tool for performing data quality checks on OSV records, individually and in aggregate.
+
+## Vision
+
+* OSV record producers are able to run a tool on records they publish, as part of their record publishing pipeline, to discover any data quality issues that might negatively impact the utility of the record by downstream users
+* OSV.dev has a repeatable and transparent mechanism for maintaining a quality bar above and beyond what JSON Schema validation enables
+* Interested parties can contribute additional checks
+* Interested parties may conduct analyses across OSV records in aggregate
+
+Inspired by [github.com/mprpic/cvelint](https://github.com/mprpic/cvelint), and under active development.
+
+### TODO
+
+* Define and implement a machine-readable format for check results to facilitate integration
+  * Today, a non-zero return code is the coarse-grained indicator of correctness
+* Add a flag to specify checks to be ignored
+
+## Usage
+
+```text
+$ go run ./cmd/osv/ record lint --help
+NAME:
+   osv record lint - check OSV records for correctness
+
+USAGE:
+   osv record lint [command options]
+
+OPTIONS:
+   --verbose                                  verbose output (default: false)
+   --collection value                         check collection to use (use 'list' to see) (default: "ALL")
+   --checks value [ --checks value ]          explicitly run a specific check (use 'list' to see)
+   --ecosystems value [ --ecosystems value ]  the ecosystems to constrain package checks to (use 'list' to see)
+   --help, -h                                 show help
+```
+
+```text
+$ go run ./cmd/osv record lint test_data
+test_data/nointroduced-CVE-2023-41045.json:
+         * [R0001]: missing 'introduced' object in event
+test_data/nondistinct-CVE-2018-5407.json:
+         * [R0002]: overlapping event: "e818b74be2170fbe957a07b0da4401c2b694b3b8"
+2024/10/22 00:04:23 found errors
+exit status 1
+```
+
+```text
+$ go run ./cmd/osv/ record lint --checks list
+Available checks:
+
+A0001: (affected-data-exists): every record has affected data
+R0001: (introduced-event-exists): every range has an introduced event
+R0002: (range-is-distinct): range spans multiple versions/commits
+P0001: (package-exists): package exists in ecosystem's registry
+P0002: (package-versions-exist): package versions exist in ecosystem's registry
+P0003: (package-purl-valid): package purl validates
+```
+
+```text
+$ go run ./cmd/osv/ record lint --collection list
+Available check collections:
+
+ALL: all checks currently defined
+        A0001: (affected-data-exists): every record has affected data
+        R0001: (introduced-event-exists): every range has an introduced event
+        R0002: (range-is-distinct): range spans multiple versions/commits
+        P0001: (package-exists): package exists in ecosystem's registry
+        P0002: (package-versions-exist): package versions exist in ecosystem's registry
+        P0003: (package-purl-valid): package purl validates
+offline: checks that do not have remote data dependencies
+        A0001: (affected-data-exists): every record has affected data
+        R0001: (introduced-event-exists): every range has an introduced event
+        R0002: (range-is-distinct): range spans multiple versions/commits
+        P0003: (package-purl-valid): package purl validates
+```
+
+## Contributing
+
+Contributions are very welcome!
+
+Checks should be as atomic as possible.
+
+### Adding checks
+
+* Define in `internal/checks`, based on the nature of the check
+  * `packages.go`
+  * `ranges.go`
+  * `record.go`
+  * Define a new variable of type `&CheckDef`, in the name format `Check` + *thing* + *assertion* (where *thing* is field of an OSV record and *assertion* is what is being checked)
+  * Implement a function that takes a `*gjson.Result` and a `*Config` and returns `[]CheckError`
+    * Include tests and sample records that both pass and fail this check
+* Add the new `&CheckDef` variable to `Collections` in `internal/checks/checks.go`
+* See [#295](https://github.com/ossf/osv-schema/pull/295) for a worked example
+
+### Additional references
+
+* [Open Source Vulnerability schema](https://ossf.github.io/osv-schema/)
+* [Properties of a High Quality OSV Record](https://google.github.io/osv.dev/data_quality.html)
+* [GJSON](https://github.com/tidwall/gjson)
+  * [Go package](https://pkg.go.dev/github.com/tidwall/gjson)
+  * [Syntax](https://github.com/tidwall/gjson/blob/master/SYNTAX.md)
+  * [Playground](https://gjson.dev/)