Consider adding credentials example for in-memory key to documentation

[thesurlydev]

The current docs (https://vanniktech.github.io/gradle-maven-publish-plugin/central/#secrets) only show an example of setting credentials when using a secretKeyRingFile.

I found that when using an exported in-memory key, the following works:

signingInMemoryKey=lQdGBF4jUfwBEtEpr64iB9b6YRkWil3EODiiLd9JS3V...9pip+B1QLwEdLCEJA+3IIiw4qM5hnMw=
signingInMemoryKeyId=your_key_id
signingInMemoryKeyPassword=your_password

It wasn't until I looked at the source code that I realized this was possible.

[iRYO400]

@thesurlydev I found out that too, but that keys doesn't work on Github Actions, because GA's secrets are not case sensetive. I'm trapped :(

[gabrielittner]

It's described in the environment variables tab on the page you linked to. The idea was that the in memory key is probably a thing mostly used on CI. I'll completely split in memory vs keyring in the docs to make it more obvious and the in memory docs will get both a property and an env var example.