Add installation section

Editor assessment

The docs type is correct and the subject 'add installation section' is imperative and clear. Structurally, though, there are two blank lines between the header and body where the spec expects exactly one — a small deviation some parsers tolerate but strict tooling may flag. The body itself is weak: bullets like 'Implemented requirements', 'Added one-step installation', and 'Finished section' are past-tense progress notes from the PR checklist, offering little beyond the subject. A sentence describing what the installation section covers (package managers, supported platforms) would add real value. Everything essential — type, mood, length — is in order; the body is filler that could be dropped or rewritten.