Update README and description

Author: mradbourne

test/README.md

@@ -1,6 +1,19 @@
 The tests in the `./tests/` directory were originally copied from the [messageformat project](https://github.com/messageformat/messageformat/tree/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/__fixtures)
 and are here relicensed by their original author (Eemeli Aro) under the Unicode License.
 
+These test files are intended to be useful for testing multiple different message processors in different ways:
+
+- `syntax.json` — Test cases that do not depend on any registry definitions.
+
+- `syntax-errors.json` — Strings that should produce a Syntax Error when parsed.
+
+- `data-model-errors.json` - Strings that should produce Data Model Error when processed.
+  Error names are defined in ["MessageFormat 2.0 Errors"](../spec/errors.md) in the spec.
+
+- `functions/` — Test cases that correspond to built-in functions.
+  The behaviour of the built-in formatters is implementation-specific so the `exp` field is often
+  omitted and assertions are made on error cases.
+
 Some examples of test harnesses using these tests, from the source repository:
 
 - [CST parse/stringify tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/cst/cst.test.ts)

test/tests/data-model-errors.json

@@ -1,6 +1,6 @@
 {
   "scenario": "Data model errors",
-  "description": "Each `src` should produce an error when processed. each test description indicates the error reason, but this text should not be asserted against. Error names are defined in the spec.",
+  "description": "Each `src` should produce an error when processed. Each test description indicates the error reason, but this text should not be asserted against. Error names are defined in the spec.",
   "defaultTestProperties": {
     "testType": "syntax",
     "locale": "en-US",