Low memory tip for building

[Jakeisthesnake]

[Disclaimer - I am a complete beginner, so feel free to reject my pull request.]

I was finally able to build the moveit and moveit tutorials packages using: MAKEFLAGS="-j4 -l1" colcon build --executor sequential

I am not sure why those make flags worked for me, since as far as I could tell from documentation, --executor sequential limits the build to one job at a time. But it worked for me, so I thought I would post it in case anyone else finds it helpful.

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