Add a missing log4rs badge to intro

[cschneid]

The "Log messages to a custom location" item on the intro page had only
the log badge, but was missing the log4rs badge. The log4rs badge
was already present on the recipe.

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Things to check before submitting a PR

• the tests are passing locally with cargo test
• cookbook renders correctly in mdbook serve -o
• commits are squashed into one and rebased to latest master
• PR contains correct "fixes #ISSUE_ID" clause to autoclose the issue once PR is merged
• non rendered items are in sorted order (links, reference, identifiers, Cargo.toml)
• links to docs.rs have wildcard version
• code identifiers in description are in hyperlinked backticks

[`Entry::unpack`]: https://docs.rs/tar/*/tar/struct.Entry.html#method.unpack

Things to do after submitting PR

• check if CI is happy with your PR
• drop a comment on #209 if you consent to repository being relicensed with CC0 license 👍

[budziq]

@cschneid Thanks!

Please drop a comment on #209 if you consent to repository being relicensed with CC0 license!

[budziq]

I also noticed that the table in logging.md also misses the log4rs badge would you consider making the PR here too?