Add a note in README.md regarding pre-import of backend

[SuryanarayanaY]

With keras_core users has to import the backend first before importing keras_core. It has to be documented in README.md for better visibility and reach to the community.

Fixes #833

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (4b7ede4) 76.21% compared to head (e72b90f) 76.21%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #863 +/- ## ======================================= Coverage 76.21% 76.21% ======================================= Files 329 329 Lines 31270 31270 Branches 6083 6083 ======================================= Hits 23833 23833 Misses 5844 5844 Partials 1593 1593 ``` | [Flag](https://app.codecov.io/gh/keras-team/keras-core/pull/863/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | Coverage Δ | | |---|---|---| | [keras_core](https://app.codecov.io/gh/keras-team/keras-core/pull/863/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | `76.12% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team#carryforward-flags-in-the-pull-request-comment) to find out more.

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