Update installation instructions

Add a reminder to upgrade packages and add a line break to improve
readability (make a line code as code appearance).
6 jobs for update-readme in 0 seconds (queued for 745 minutes and 5 seconds)
Name Stage Failure
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ubuntu_16.04_x86_64_deb Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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raspbian_stretch_armv7l_deb Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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debian_stretch_x86_64_deb Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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windows_i386_innosetup Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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osx_10.8_x86_64_dmg Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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osx_10.11_x86_64_dmg Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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