readme: update no_std notes in readme; ci: add no_std test

[hsqStephenZhang]

background

before(include) 0.2.8, it won't compile on no_std target with the following config:

caches = { version = "0.2.8" ,default-features = false, features = ["hashbrown"] }

since 0.2.9 is not published, the current work around is:

caches = { git = "https://github.com/al8n/caches-rs.git", branch = "main", default-features = false, features = ["hashbrown", "libm"] }

this should also be pointed out in readme
This pull request includes updates to the CI workflow and documentation to improve testing and clarify feature usage for the caches crate.

CI Workflow Updates:

• .github/workflows/ci.yml: Added a new test step to run tests with no_std, hashbrown, and libm features enabled.

Documentation Updates:

• README.md: Updated the no_std section to clarify the compilation requirements for versions before and after 0.2.8, and provided instructions for using the hashbrown and libm features with version 0.2.9 or the main branch from the repository.

todo

• publish 0.2.9 so it can work with no_std

[al8n]

I just bumped up some versions, and increased the version to 0.3.x, are you willing to update this PR? I will publish 0.3.x once this PR is merged.

[hsqStephenZhang]

yeah, i'd be glad to