docs: add installation option and the OpenUPM badge

[favoyang]

Hi @pschraut,

Thanks for making this awesome UPM package. I'm the creator of the OpenUPM platform, an open-source UPM registry, and continuous build service based on Git tags. It's hosting more than 200+ open-source UPM packages at the time of writing.

Your package has been submitted to the platform, and it works well. https://openupm.com/packages/com.oddworm.texture2darrayimportpipeline/. Please consider merging the docs update PR.

If this makes sense for you, feel free to add the badge (from the package detail page right bottom) to other your UPM packages.

• https://openupm.com/packages/com.oddworm.texture3datlasimportpipeline/
• https://openupm.com/packages/com.oddworm.cubemaparrayimportpipeline/
• https://openupm.com/packages/com.oddworm.dbgdraw/