Checking for previously installed APM or improve documentation

[frnco]

Following the instructions to build from source on Linux, there was no mention of either how to use an existing apm installation or the fact that atom will install its own apm version.

How I ran on this problem:
I had manually installed apm, since apm was not working when installed through pacman (The PACkage MANager for Arch Linux). I confirmed the issue is not with APM, it worked fine, so I went on to also install atom from the sources. Right after calling script/bootstrap as per Atom's building instructions, Atom printed a line telling me it was installing APM.

What exactly I believe is a problem:
There was no information easily noticeable about a way to use an existing apm installation, and no warnings that it is impossible to use an existing APM installation, which left me with lots of questions. Sure this may seem trivial but to point one undesired effect, I did a quick du -hd1 on bot the folder of the APM installation I alreade had, and on the apm folder on the folder in which I installed Atom, and got 186Mb on the one in the atom folder, and 294Mb on the one I already had.

I can elaborate if it's desirable, but being very direct:

• This can be misleading to users that might unaware they have one useless copy of apm installed if they don't pay attention to the output;
• Someone may care about those 186Mb (Actually I kinda do care, though only sometimes)

Case in point: I was surprised, in a bad way, by the behavior, and it can be fixed with a simple change on the documentation, be it adding how to specify an existing APM installation, or be it adding one line saying APM is bundled and you can only use the bundled version period.

I believe people should know what's going on in their computers, so I probably care more about this than most people, but I'm proof that there are people who care about this so I believe it's worth clarifying in the docs, especially since it'll mean adding at most a few lines, possibly a single one.