building info for raspi 3B+

others may add notes for other platforms

Building.md

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+Raspberry Pi 3B+
+---------------
+
+Building more than ubxtool, navdump, and navparse will require quite a bit of available RAM. 
+In the Raspbian image, the default swap file size for _/var/swap_ is only 100MB.
+More swap space will be needed if you want the compiling to finish without it seeming
+to take a geologic age.
+
+The swap file is configured in _/etc/dphys-swapfile_ and managed using the __/sbin/dphys-swapfile__ utility.
+
+Assuming the output of _df -h_ and _free_ indicate you have sufficient free file space and
+the swap file is indeed 100MB (respectively), and you do not have Chromium running (it eats
+RAM), you edit the config file (suggest changing 100 to 960) and then (as root):
+```
+/sbin/dphys-swapfile swapoff
+echo Be Patient. This could take a while
+/sbin/dphys-swapfile setup
+/sbin/dphys-swapfile swapon
+```