Cleanup of README.md

[ckaran]

I noticed that the README.md file had a number of minor issues that meant that it wasn't as clear as it could be. This PR clears them up.

Issues

• I rewrapped all lines to 80 columns. This makes it a little easier to read in a text editor.
• I added bash as the info string of every fenced code block. This makes Github render the blocks correctly, and makes copying simpler.
• I added two new environment variables to the prerequisites. These are ROS1_INSTALL_PATH, and ROS2_INSTALL_PATH. They exist to make it simple to run the scripts regardless of which versions of ROS 1 and 2 are currently in use. Instructions on how to set them are given within the README.md

[gbiggs]

Text (and similar, e.g. Markdown) files in ROS 2 are intentionally not wrapped. We use one-sentence-per-line because it makes diffs in PRs much easier to handle.

[ckaran]

Understood. I'll unwrap everything and resubmit.

[ckaran]

@gbiggs I've fixed the wrapping as requested.

[ckaran]

@gbiggs Thanks for suggestions! I had to do a force push to signoff the commits and get them past the DCO bot, I apologize if you need to do something on your end to resync with my commits.

[ckaran]

OK, I modified 'galactic' -> 'rolling'.

[gbiggs]

Thanks for the contribution!

[ckaran]

You're welcome!