Write down the project's goals, non-goals, and roadmap

Several decisions in here look like oversights until you know the
reasoning: the curated model subset with everything else in `extra`, the
still-generic `chart`, not retrying `POST`, the deliberate divergences
from the official PHP SDK. That reasoning was spread across code
comments and commit messages, where it is easy to miss and easy to
"fix".

GOALS.md states:

- What the crate is for: complete coverage, and not quietly losing data,
  truncating results, or succeeding when it did not.
- Seven goals, each tied to a decision visible in the code, so the
  rationale survives the next refactor.
- Non-goals, including no sync API, no caching or ORM layer, no
  modelling of every field, and no retrying of non-idempotent requests.
- Publication posture: on crates.io, pre-1.0, no stability promise, pin
  an exact version. MSRV may rise with dependencies.
- A roadmap ordering the COVERAGE.md gaps by value over effort, starting
  with multipart uploads, which is the only thing blocking
  `create_document`.
- Open questions, left open rather than papered over: whether auth routes
  belong here at all given they are a different auth model, whether
  per-chart types earn their keep, streaming for large exports, and the
  untested `Error::PaginationLimit`.

README, COVERAGE.md, and CLAUDE.md link to it. No changelog entry: this
adds no behaviour, per the rule in CLAUDE.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

COVERAGE.md

@@ -2,6 +2,10 @@
 
 What this crate wraps of the Invoice Ninja v5 API, and what it doesn't.
 
+Full coverage is a stated goal, so the gaps below are a to-do list rather
+than a permanent boundary — [`GOALS.md`](GOALS.md) puts them in rough
+priority order.
+
 Checked against `routes/api.php` on the `v5-stable` branch of the
 [invoiceninja repo](https://github.com/invoiceninja/invoiceninja), which
 declares 34 `Route::resource` groups and 274 explicit route declarations