[ES UI Shared] Added README (#72034)

* Added readme to es-ui-shared

* implement PR feedback; clarify terms and tighten grammar

* added note about intended users of es ui shared modules

src/plugins/es_ui_shared/README.md

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+## ES UI shared modules
+
+This plugin contains reusable code in the form of self-contained modules (or libraries). Each of these modules exports a set of functionality relevant to the domain of the module.
+
+**Please note**: Modules in ES UI shared are intended for use by the ES UI Management Team (elastic/es-ui@) only. Please reach out to us if there is something you would like to contribute or use in these modules.
+
+## Files and folders overview
+
+- `./public` | `./server`. Folders for grouping server or public code according to the Kibana plugin pattern.
+- `./__packages_do_not_import__` is where actual functionality is kept. This enables modules more control over what functionality is directly exported and prevents parts of modules to be depended on externally in unintended ways.
+- `./public/index.ts` | `./server/index.ts` These files export modules (simple JavaScript objects). For example, `Monaco` is the name of a module. In this way, modules namespace all of their exports and do not have to be concerned about name collisions from other modules.
+
+## Conventions for adding code
+
+When adding new functionality, look at the folders in `./__packages_do_not_import__` and consider whether your functionality falls into any of those modules.
+
+If it does not, you should create a module and expose it to public or server code (or both) following the conventions described above.
+
+### Example
+
+If I wanted to add functionality for calculating a Fibonacci sequence browser-side one would do the following:
+
+1. Create a folder `./__packages_do_not_import__/math`. The name of the folder should be a snake_case version of the module name. In this case `Math` -> `math`. Another case, `IndexManagement` -> `index_management`.
+2. Write your function in `./__packages_do_not_import__/math/calculate_fibonacci.ts`, adding any relevant tests in the same folder.
+3. Export functionality intended _for consumers_ from `./__packages_do_not_import__/math/index.ts`.
+4. Create a folder `./public/math`.
+5. Export all functionality from `./__packages_do_not_import__/math` in `./public/math/index.ts`.
+6. In `./public/index.ts` import `./public/math` using `import * as Math from './public/math;`. The name (`Math`) given here is really important and will be what consumers depend on.
+7. Add the `Math` module to the list of exported modules in `./public/index.ts`, e.g. `export { <...other modules>, Math }`
+8. Use `Math` in your public side code elsewhere!
+
+This example assumes no other appropriate home for such a function exists.