Set apart group membership change from udev rules file

[pierre-haessig]

Hi,

In the present state of the doc on 99-platformio-udev.rules, it is suggested that, after adding the udev rules file, it may be necessary to also change group membership (adding its own user to dialout or uucp group).

Now, unless I miss something, this is more of an alternative:

• either add the udev rules file to grant write access to all users
• or add its own user to the appropriate group

I've updated the doc in that sense (keeping the udev rule first, and splitting the group change stuff afterwards), also adding a paragraph to guide the reader in finding out the appropriate group name, since it may be board and distro dependent.

Remark: I hope I didn't made too many syntax errors, because I only had access to a text editor with reST syntax highlighting (better than nothing), but I don't have the setup to recompile the doc.

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[ivankravets]

Thanks for the PR!