Added a line about interactive usage with mathlib
Added a line about interactive usage with mathlib

Usually `bc` isn't very useful if it doesn't return have floating point number where appropriate. e.g., 3/4 returns 0 when started with `-i` but for most users they would expect 0.75 to be returned so the argument `-l`does what the user would generally want.
deekim/tldr,zoisom/tldr,zoisom/tldr,felixonmars/tldr,katekyan/tldr,UncommonPleb/tldr,UncommonPleb/tldr,Phizho/tldr,Phizho/tldr,deekim/tldr,zoisom/tldr,cptdeka10/tldr,felixonmars/tldr,felixonmars/tldr,cptdeka10/tldr,deekim/tldr,slash3b/tldr,katekyan/tldr,katekyan/tldr,Phizho/tldr,slash3b/tldr,slash3b/tldr,UncommonPleb/tldr,cptdeka10/tldr