Add footnote to niche_numerics/float to explain that you dont need to optimize unnecessarily

[legoaggelos]

https://javabook.mccue.dev/niche_numerics/float
opportunity to add a footnote at "trying to save space in memory". sth along the lines of "dont get crazy with it, a few doubles wont take up any noticable space"
cause when a beginner reads sth like that they often go crazy with what they just learned and replace all their doubles with floats, all their ints with shorts and so on

originally suggested in the TJ discord by zabuzard.