ReadMe: Add git-lfs to build instructions

[philipphofmann]

Description

When not having git-lfs installed git clone fails with

Cloning into 'woocommerce-ios'...
remote: Enumerating objects: 8114, done.
remote: Counting objects: 100% (8114/8114), done.
remote: Compressing objects: 100% (6576/6576), done.
remote: Total 8114 (delta 2118), reused 3660 (delta 1048), pack-reused 0 (from 0)
Receiving objects: 100% (8114/8114), 54.01 MiB | 16.65 MiB/s, done.
Resolving deltas: 100% (2118/2118), done.
git-lfs filter-process: git-lfs: command not found
fatal: the remote end hung up unexpectedly
warning: Clone succeeded, but checkout failed.
You can inspect what was checked out with 'git status'
and retry with 'git restore --source=HEAD :/'

Steps to reproduce

Call git clone when not having git-lfs installed.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.