Add README.md. (#28948)

x-pack/plugins/uptime/README.md

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+# Uptime Monitoring
+
+## Purpose
+
+The purpose of this plugin is to provide users of Heartbeat more visibility of what's happening
+in their infrasturcture. It's primarily built using React and Apollo's GraphQL tools.
+
+## Layout
+
+There are three sections to the app, `common`, `public`, and `server`.
+
+### common
+
+Contains GraphQL types, constants and a few other files.
+
+### public
+
+Components come in two main types, queries and functional. Queries are extended from Apollo's queries
+type which abstracts a lot of the GraphQL connectivity away. Functional are dumb components that
+don't store any state.
+
+The `lib` directory controls bootstrapping code and adapter types.
+
+There is a `pages` directory; each view gets its own page component.
+
+The principal structure of the app is stored in `uptime_app.tsx`.
+
+### server
+
+There is a `graphql` directory which contains the resolvers, schema files, and constants.
+
+The `lib` directory contains `adapters`, which are connections to external resources like Kibana
+Server, Elasticsearch, etc. In addition, it contains domains, which are libraries that provide
+functionality via adapters.
+
+There's also a `rest_api` folder that defines the structure of the RESTful API endpoints.
+
+## Testing
+
+### Unit tests
+
+From `~/kibana/x-pack`, run `node scripts/jest.js`.
+
+### Functional tests
+
+In one shell, from **~/kibana/x-pack**:
+`node scripts/functional_tests-server.js`
+
+In another shell, from **~kibana/x-pack**:
+`node ../scripts/functional_test_runner.js --grep="{TEST_NAME}"`.
+
+### API tests
+
+In one shell, from **~/kibana/x-pack**:
+`node scripts/functional_tests-server.js`
+
+In another shell, from **~kibana/x-pack**:
+`node ../scripts/functional_test_runner.js --config test/api_integration/config.js --grep="{TEST_NAME}"`.