Extend README

[sgasse]

• Add loading of http_archive rule.
• Explicilty mention to replace commit hashes.

[chuckx]

This removes a necessary detail when dealing with release tags instead of commit hashes.

For example, take the latest release from the pybind repository (v2.5.0):

• The release tag is "v2.5.0"
• The download URLs are https://github.com/pybind/pybind11/archive/v2.5.0.zip or https://github.com/pybind/pybind11/archive/v2.5.0.tar.gz
• Within both archive formats, the project is in a directory named "pybind11-2.5.0" (note the lack of a "v" prefix)

Naming the placeholder stable-version is meant to imply the use of a release tag, i.e. for version 2.5.0 use "v2.5.0" in the urls value and "2.5.0" in the strip-prefix value.

Perhaps a better approach would be to use a a generic name for the placeholder (e.g. version-identifier) and then provide a brief explanation to point out the details of using a release tag versus a commit hash.

Note that this distinction isn't necessary for this repository. Since there are no tagged releases (for now at least), commit hashes are the only option. Hence the placeholder being named stable-commit.

[sgasse]

Ah thank you for explaining the difference, @chuckx ! I updated my changes accordingly. Would it make sense like this?