Update README to explain how to use data source for custom domain

[mikhail-vl]

1. Resolves Cannot install plugin to k8s instance of grafana #16.
2. Resolves I don't see data source after restart #17.

[github-actions]

Levitate is-compatible report:

🔍 Resolving @grafana/data@latest...
🔍 Resolving @grafana/ui@latest...
🔍 Resolving @grafana/runtime@latest...
🔍 Resolving @grafana/schema@latest...
🔍 Resolving @grafana/e2e-selectors@latest...
🔍 Resolving @grafana/experimental@latest...

🔬 Checking compatibility between ./src/module.ts and @grafana/data@9.2.6...
✔ Found @grafana/data version 9.2.2 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/ui@9.2.6...
✔ Found @grafana/ui version 9.2.2 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/runtime@9.2.6...
✔ Found @grafana/runtime version 9.2.2 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/schema@9.2.6...
Skipping package @grafana/schema because it is not used in the project or not installed locally.
did you forget to run yarn install or npm install?

🔬 Checking compatibility between ./src/module.ts and @grafana/e2e-selectors@9.2.6...
Skipping package @grafana/e2e-selectors because it is not used in the project or not installed locally.
did you forget to run yarn install or npm install?

🔬 Checking compatibility between ./src/module.ts and @grafana/experimental@1.0.2...
Skipping package @grafana/experimental because it is not used in the project or not installed locally.
did you forget to run yarn install or npm install?

✔️ ./src/module.ts appears to be compatible with @grafana/data,@grafana/ui,@grafana/runtime,@grafana/schema,@grafana/e2e-selectors,@grafana/experimental

[codecov]

Codecov Report

Base: 89.47% // Head: 89.47% // No change to project coverage 👍

Coverage data is based on head (c6d7ca3) compared to base (35f1397).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #18   +/-   ##
=======================================
  Coverage   89.47%   89.47%           
=======================================
  Files          10       10           
  Lines         133      133           
  Branches        3        3           
=======================================
  Hits          119      119           
  Misses         13       13           
  Partials        1        1           

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[mikhail-vl]

Looks good!