docs: add order requirements section

[dr-gideon]

Overview
Adds an "Order requirements" section to the README documenting minimum order size, tick size, and valid price range. Addresses user confusion reported in #301.

Description
Added a new section between "Place a limit order" and "Manage orders" that documents:

Minimum order size (5 shares)
Minimum tick size (0.01)
Valid price range (0.01–0.99)
A common pitfall note explaining why small dollar amounts at high prices get rejected

Types of changes
Other, additional

Notes
Fixes #301 - users placing small dollar orders are getting Size lower than the minimum: 5 errors with no explanation in the docs.

Status
Prefix PR title with [WIP] if necessary (changes not yet made).
Add tests to cover changes as needed.
Update documentation/changelog as needed.
Verify all tests run correctly in CI and pass.
Ready for review/merge.

---

Note

Low Risk
Low risk documentation-only change; no runtime code paths are modified.

Overview
Adds an "Order requirements" section to README.md describing the minimum order size (5 shares), minimum tick size (0.01), valid price range (0.01–0.99), and a note on small-$ orders at high prices causing Size lower than the minimum: 5 rejections.

Also fixes a minor README formatting issue at the end of the file (removes a stray leading - and restores the trailing newline).

^{Written by Cursor Bugbot for commit 72da864. This will update automatically on new commits. Configure here.}

[RecRanger]

This is a really good piece of documentation to include. It should realistically also go in the upstream REST API docs, as I think most people using the APIs have to find this out the hard way (manually).