Optimize the building script by making a shallow clone

[pasabanov]

Brief

This pull request includes a small change to the .github/README.md file. The change modifies the git clone command to use the --depth 1 option, which reduces the clone size by only fetching the latest commit.

This reduces the download time from approximately 1.05s user 0.23s system 65% cpu 1.947 total to approximately user 0.04s system 10% cpu 0.999 total in my case and the download size from 7.6 megabytes to 540 kilobytes.

[iskunk]

Hi @pasabanov,

I appreciate the doc update, but note that the doc is for a dead process. There is no current version of ungoogled-chromium that can be built in the manner described in that README; the only up-to-date means available here is that under the convert/ subdirectory.