docs: add empty CHANGELOG.md placeholder (#16)

Makes the CHANGELOG.md link added to README.md (#15) resolve to a real
file instead of a 404, ahead of the first real release-please run
creating it for real.

Verified this is safe by reading release-please's actual updater source
(`updaters/changelog.js` in the npm package): content is coerced with
`content || ''`, and an empty string is falsy, so a 0-byte file takes
the exact same code path as a genuinely absent file — clean header +
first entry, nothing demoted or duplicated. Any non-empty placeholder
text, by contrast, survives the bootstrap but gets pushed below the
first release entry as an orphaned, permanently-demoted sub-heading —
confirmed from the same source, not worth risking.

No conflict with #15 (different file) — can merge in either order.

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

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: dconlan

CHANGELOG.md

@@ -0,0 +1,32 @@
+# Changelog
+
+## [0.5] - 2024-02-23
+- Add support for a LOINC FHIR server
+- Change the stored value format to `code|system` (drop `display`), avoiding failures when long display text pushed values past REDCap's field length limit
+
+## [0.4] - 2022-09-06
+- Add basic `@HIDECHOICE` support (hide specific codes from a field's autocomplete results)
+- Add a `User-Agent` header, since some FHIR servers (e.g. SNOMED's) reject requests without one
+- Work around an `http_post` bug where a custom header combined with a custom content type caused the content type to be silently overwritten
+- Recommend the `tx.ontoserver.csiro.au` server over the R4 server in documentation
+
+## [0.3] - 2021-11-19
+- Add support for an authenticated FHIR terminology server
+- Add a web service so the Online Designer talks to REDCap, not directly to the FHIR server - protects authentication settings and allows the module to work behind a proxy
+
+## [0.2.3] - 2020-02-03
+- Fix a null `$project_id` handling issue in a hook
+
+## [0.2.2] - 2019-08-05
+- Fix `redcap_csrf_token` being incorrectly added to AJAX JSON POST requests
+
+## [0.2.1] - 2019-01-29
+- Fix a hook function signature incompatibility when `null` is passed
+
+## [0.2] - 2019-01-25
+- Add a tooltip mechanism for selected values
+- Add an option to return a predefined value instead of an empty result set
+- Add an indication when no search results are found
+
+## [0.1] - 2018-11-23
+- Initial release