Record the preserving writer in the changelog, for v0.3.0-rc.1

[samuelduchesne]

The writer landed in #45 with no changelog entry. This is that entry, ahead of cutting v0.3.0-rc.1.

A minor bump rather than a patch, matching what v0.2.0 did for the syntax layer: this adds public API — preserveFormatting across five entry points and IdfDocument.rawText — and breaks nothing. A patch number on a 0.x package reads as 'fixes only' to anyone looking at the version alone.

The entry records the three things a reader would otherwise have to discover:

• the object notation preserves all-or-nothing, unlike the text format's per-object terms, and does so identically in Python;
• extensible repeats become accessors, and only on a preserving document, because reading a coordinate through one costs about 33x a plain read and a document with no retained source has no record to keep;
• the one spelling the wrapper cannot hear, replacing a whole repeat by index, with the two tracked alternatives beside it.