`workspace` input is not documented in README.md

[mmaetzler]

When adding the workspace flag (which is very helpful for projects using cocoapods) to this action, there were two PRs:

• Add a workspace input #80
• Add workspace as an input configuration #81

#80 got merged and #81 was closed without merging. However, #81 had an update to the README.md file to introduce the newly introduced workspace flag which did not make it into master.

Since this is a very useful flag, I think the documentation for it should be added to the README.md

[mmaetzler]

I went ahead and created a PR for this issues, since it was easy to just copy the previous text from #81 over. I hope that works.