docs: add system-wide installation step to README

[Copilot]

The README lacked installation instructions. It already covered dependencies and building, but the make install target (which installs the snake binary and high_score.txt data file via CMakeLists.txt) was undocumented.

Changes

• README.md: Added step 6 to the "Building the Game" section documenting the optional sudo make install command, including install destinations for the binary (/usr/local/bin) and high score file (/usr/local/share/snake/).

Original prompt

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This section details on the original issue you should resolve

<issue_title>update README.md</issue_title>
<issue_description>README needs to have instruccions for installation.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes update README.md #23

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