After speaking to @diox in the #django-compressor channel on irc, I decided to contribute a changelog. I'm not sure what changes are backwards incompatible, nor how granular you would like the changelog to be, but here's a start anyway.

I generated a list of the PRs that have been merged in post-1.3 by doing the following:

$ pip install nodeenv
$ nodeenv -p
$ npm install -g github-changes
$ github-changes -o django-compressor -r django-compressor -a -b develop  --only-pulls --use-commit-body

This generated a file containing the following, which I have attempted to convert into the changelog.txt file in the docs folder:

upcoming (1.4) (2014/04/23 23:32 +00:00)

• #312 Fixed CompilerFilter to always output Unicode strings (@anttihirvonen)
• #382 Clarifying default for COMPRESS_CACHE_BACKEND (@alecxe)
• #406 Fix Tests fail with latest version of html5lib #405 - compatibility with html5lib 1.0 (@ambv)
• #402 Set timestamps of files written by GzipCompressorFileStorage (@bgilbert)
• #387 Add make target for setting up test environment (@codeinthehole)
• #439 add usedevelop to tox testenv (@RonnyPfannschmidt)
• #463 Fix for extracting version number in Python 3 (@maikhoepfel)
• #451 Configure tox and requirements to test with django 1.6 (@diox)
• #427 Make CssCompressor class easier to extend (@jor123)
• #453 Support for the kind in the endblock (@wolph)
• #456 Allow data-uri filter to handle external/protocol-relative references (@shacker)
• #447 Fix windows line endings in offline compress (@apmorton)
• #416 Don't raise an error if BeautifulSoup isn't installed (@damianmoore)
• #444 + add rcssmin (@Mystic-Mirage)
• #464 -Fix: compare against six.string_types. (@Xaroth)
• #437 change DEBUG_TOGGLE default value from 'None' to None (@fsw)
• #469 Fix README.rst to show percentage instead of 'unknown'. (@iknite)
• #472 Try import django.utils.simplejson, except import json (@agriffis)
• #474 Fixed grammar in Readme (@vishalsodani)
• #468 Tests fail with libxml-2.9.1 (@agriffis)
• #473 Spelling fixes in README.rst (@entrepreneurj)
• #478 Fix: Install instructions don't work with the new default project template #477 Enhanced Error display if COMPRESS_ROOT nor STATIC_ROOT is defined (@iknite)
• #480 Fix Install instructions don't work with the new default project template #477: Add install instructions for COMPRESS_ROOT and STATIC_ROOT. (@iknite)
• #494 Spelling correction in docs/behind-the-scenes.txt (@eric-fish)
• #488 Offline compression for Jinja2 with Jingo and Coffin integration (@lucastan)
• #489 Document settings.COMPRESS_CACHE_KEY_FUNCTION (@adamjforster)
• #498 Relax the test for a configured CachedLoader. Fixes No template paths found #493. (@mikebryant)