Closes#3.
- doc.go: package overview -- what a validator is, All vs Collect, errors
are sentinels, how this compares to validator/v10 (tags+reflection),
ozzo-validation (Rule interface) and govalidator (IsX helpers), and when
to pick something else.
- README.md: rewritten from three lines to a positioning intro, a
"why this and not X" table, a building-blocks table, and a
"not a fit if" note.
- example_test.go: runnable Example (compose built-ins + a closure),
Example_collectAll (errors.Is over a joined error), Example_errorsIs
(sentinel matching, Err as match-anything).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
