Add README with install and usage guide

Signed-off-by: Snorre Magnus Davøen <snorre.davoen@nsd.uib.no>

README.md

-# Log Schema Node
\ No newline at end of file
+# Log Schema Node
+
+This is a Node module intended to make the
+[nsd-commons/log-schema](https://gitlab.nsd.uib.no/nsd-commons/log-schema)
+consumeable by Node applications. It exposes the
+[JSON Schema](http://json-schema.org/) based log schema as a Javascript object,
+and a corresponding Typescript definitions file.
+
+
+## Installation
+
+If you use npm:
+```bash
+# Add nexus.nsd.no npm repo if not already added:
+npm set registry https://nexus.nsd.no/repository/npm-group/
+npm install --save @nsd/log-schema-node
+```
+
+If you use yarn:
+```bash
+# Add nexus.nsd.no npm repo if not already added:
+npm set registry https://nexus.nsd.no/repository/npm-group/
+yarn add @nsd/log-schema-node
+```
+
+## Usage
+
+Typescript example below. ES6 would look much the same.
+```typescript
+import {LogSchema, schema} from "@nsd/log-schema-node";
+import * as JaySchema from "jayschema";
+
+const js = new JaySchema();
+
+const logEvent: LogSchema = {
+    schemaVersion: "v3",
+    "@timestamp": "1985-04-12T23:20:50.52Z",
+    levelName: "INFO",
+    message: "Ørliten melding",
+    serviceName: "some-service",
+    serviceVersion: "v1.0.0"
+};
+
+console.log(js.validate(logEvent, schema));
+```
+
+Currently this package only exposes the schema and the Typescript interface. In
+the future it might also provide facilities for validation, etc.