[ui] Add a brief readme to dagster-ui (#23913)

## Summary & Motivation

We don't really describe what steps we would like external contributors
to take when they put up pull requests. Here's a basic initial readme to
give a bit of guidance.

## How I Tested These Changes

Preview

## Changelog [New | Bug | Docs]

NOCHANGELOG

js_modules/dagster-ui/README.md

@@ -0,0 +1,22 @@
+## Dagster UI
+
+The Dagster frontend application is built with TypeScript and React.
+
+This directory contains the following packages:
+
+- `app-oss`: A thin Next.js wrapper that serves as the root of the app.
+- `ui-core`: The main components and routes of the application.
+- `ui-components`: The core React component library, representing our design system
+- `eslint-config`: A custom eslint configuration used across our frontend projects.
+
+### Contributing
+
+There are a handful of commands you can run locally from the `dagster-ui` directory prior to pushing a branch for a pull request:
+
+- `make ts`: Run TypeScript checks across all packages.
+- `make lint`: Run eslint (including prettier and autofixes) across all packages.
+- `make jest`: Run Jest tests across all packages.
+
+Each `make` command runs the named command within each package directory. For instance, `make ts` will run `yarn ts` in each of the package directories.
+
+If you are making changes to GraphQL queries or fragments, you will also need to run `make generate-graphl`. This step builds new TypeScript types for queries and fragments. After regenerating GraphQL types, run `make ts` and `make lint` to find locations in the code where the updated types have resulted type and lint errors, and fix the errors prior to pushing the change.