Release 0.28.0

[Mingun]

Checklist (minimal list of actions for cutting release):

• Merge Merge consequent text and CDATA events into one string  #520
• Merge Fix using incorrect deserializer when deserialize Option values in maps and structs #548
• Merge Implement correct handling of $text fields in enums and add roundtrip tests #574 (serialization part of Implement deserialization textual content into externally tagged enum variants #541 -- I've forgot about it in the original PR)
• Merge Truly check that result is borrowed when it should #550
• Merge Document some internal macros and other small improvements #577
• $env:RUSTDOCFLAGS="--cfg docs_rs"; cargo +nightly doc --all-features and check generated documentation for missing / unclear things
• Update version in Cargo.toml
• Update Changelog.md with date of release, add new empty Unreleased headings
• cargo package for final check
• Push master to my fork, wait while CI pass. Repeat with force pushs if necessary
• cargo publish
• Create and push tag v0.28.0 and push master to upstream
• Create a Release on GitHub (in GitHub UI)

[Mingun]

@dralley, It seems, that #379 still requires some work and probably won't be finished in the near week, so I think I'll create release without it.

I also think, that I'll take some time off after relasing version and will not work on other PRs for some time. That does not means that I will not review and merge PRs. Just need a little break from quick-xml.