suggest cloning with submodules in README.md

[stevengj]

Resolves: #33517 (comment)

This updates the README.md to suggest using git clone --recurse-submodules in order to clone the MuseScore repository, which is now necessary in order to clone a buildable repository because muse_framework has become a submodule (#33302).

(Otherwise, following the build instructions leads to errors about missing cmake files, as commented here: #33517 (comment))

• I signed the CLA — user stevengj
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR updates the repository cloning instructions in README.md. The git clone command now includes the --recurse-submodules flag to automatically check out submodules during clone, and adds a parenthetical note explaining that this ensures the muse_framework submodule is placed under the muse/ directory. The previous separate git clone and cd commands without the submodule flag have been consolidated into this single instruction.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly and clearly describes the main change: updating README.md to suggest using git clone with submodules flag.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description follows the template structure with a resolved issue link, clear motivation, and most checklist items completed (5 of 8 checked). However, some checklist items are unchecked.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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