Prerequisites review reminder wrt jpm

[sogaiu]

jpm doesn't currently come with a git clone of janet's repository.

As far as I know, it's not quite clear how folks are going to be expected to install it yet.

This issue is just a reminder to revisit the prerequisites section of the README if separate installation of jpm becomes the norm.

[sogaiu]

Janet's Makefile acquired a new target named install-jpm-git. If one is building and installing Janet from the source repository, this can be used to install jpm.

One thing worth noting might be that if one uses PREFIX while building janet, it's likely important to use that for the install-jpm-git target as well.

Recently I've been using:

make clean && \
PREFIX=$HOME/.local make && \
PREFIX=$HOME/.local make install

to intall janet, so for jpm installation I would do:

PREFIX=$HOME/.local make install-jpm-git

One reason to use PREFIX like this is that it makes it straight-forward to test multiple versions of janet using multiple user accounts.

Suppose I want to test version A and version B of janet.

I can install version A of janet for user X and test there.

I can also install version B of janet for user Y and test there separarely.

So at least for people who need to investigate issues related to versions, it's much nicer to be able to have multiple installations of something within a single OS -- and currently, a relatively cheap and straight-forward way to do this is with multiple accounts if the software in question can be made to function installed in a variety of locations.

One can also use virtual machines (like VirtualBox), Docker, or NixOS / guix for this sort of thing. They all have their various trade-offs.

[saikyun]

For now I added a note that the user has to install jpm. I've removed notes about prefix for now, those were mostly for when I was replacing regular jaylib. Now freja-jaylib is a separate package. I'll link to this issue if someone wants to do it with prefixes, thanks for the details. :)