Create `imcflibs` installation instructions

[ehrenfeu]

Rationale 🤔

If someone wants to install imcflibs into a "vanilla" Fiji (i.e. freshly downloaded from fiji.sc), they'd need to know the external requirements / dependencies for using the full functionality.

One example is the docstring referred to in #60, but there is more than just that.

A good starting point would be to check out imcf/imcf-fiji-mocks and see what additional components are present there and use this to figure out:

• which update site(s) to activate or
• where to get the .jars in case they don't have a site

Additional goodies 🪅🍬

One thing that would be nice is to use this task and create additional interactive test scripts on-the-fly (cf. #83, #36), so spotting incomplete dependencies would become much easier in the future 🔮.

[CellKai]

Where should these dependencies be documented? In the README.md, or a new DEPENDENCIES.md / INSTALLATION_INSTRUCTIONS.md or elsewhere?