Change the demo link in the README.md

The demo link (https://calibre-ebook.com/demo) present in the README.md threw a certificate error. This modification makes the URL use the HTTP protocol to avoid this issue.
Merge branch 'patch-1' of https://github.com/aymericbeaumet/calibre
Fixed a libimobiledevice resource leak.
Better logo resolution
Merge branch 'patch-1' of https://github.com/sylvaindurand/calibre
Merge branch 'master' of https://github.com/GRiker/calibre
Add a note about how the beautify algorithm works.
Highlighting groups for spell check
Store language data when syntax highlighting
...
Function to return the number of utf-16 code points in a python string on wide python builds
Edit book: Fix incorrect syntax highlighting on linux if the text contains non-BMP unicode characters.
Faster implementation of utf16_length
Increase time we wait for the antivirus to curl up and die
Add a break iterator that returns indices into the string instead of tokens in the string
DOCX Input: Handle docx files with index fields that have their field names incorrectly lower cased. Fixes #1318670 [Conversion from DOCX, probably indexitem related](https://bugs.launchpad.net/calibre/+bug/1318670)
Content server: Add more descriptive titles to the OPDS feeds, where possible
Auto check for new QVariant usaeage when switching to qt5 branch
Port compiling of .ui forms
Port imports