Improve installation docs

[cndolo]

• First commit corrects the path to the venv in the top-level README.
• Second commit changes the path to the entrypoint to a perhaps more generic one. The reason for that is that the given relative path to the simln plugin might not always be right (it wasn't for me, at least). While at it, we also correct the spelling of simln to be consistent with the rest.

[bdp-DrahtBot]

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #729 (Add --capacity-multiplier option to SimLN plugin for random activity by b-l-u-e)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[pinheadmz]

Thanks!