Add note to the gotchas showing that expressions in sympy are immutable
Add note to the gotchas showing that expressions in sympy are immutable

drufat/sympy,sunny94/temp,kumarkrishna/sympy,aktech/sympy,wanglongqi/sympy,cccfran/sympy,ahhda/sympy,rahuldan/sympy,beni55/sympy,abloomston/sympy,AunShiLord/sympy,cswiercz/sympy,abhiii5459/sympy,Davidjohnwilson/sympy,Gadal/sympy,shikil/sympy,saurabhjn76/sympy,dqnykamp/sympy,drufat/sympy,kmacinnis/sympy,aktech/sympy,AkademieOlympia/sympy,mafiya69/sympy,sahilshekhawat/sympy,sahmed95/sympy,hrashk/sympy,VaibhavAgarwalVA/sympy,kmacinnis/sympy,Sumith1896/sympy,Arafatk/sympy,kaichogami/sympy,wyom/sympy,aktech/sympy,mafiya69/sympy,pbrady/sympy,asm666/sympy,rahuldan/sympy,abloomston/sympy,pbrady/sympy,cswiercz/sympy,Curious72/sympy,hrashk/sympy,souravsingh/sympy,iamutkarshtiwari/sympy,debugger22/sympy,hrashk/sympy,oliverlee/sympy,yashsharan/sympy,Mitchkoens/sympy,Curious72/sympy,diofant/diofant,atreyv/sympy,madan96/sympy,kmacinnis/sympy,kevalds51/sympy,cswiercz/sympy,kaushik94/sympy,amitjamadagni/sympy,dqnykamp/sympy,minrk/sympy,grevutiu-gabriel/sympy,kaushik94/sympy,Mitchkoens/sympy,wanglongqi/sympy,abhiii5459/sympy,grevutiu-gabriel/sympy,maniteja123/sympy,AkademieOlympia/sympy,sahmed95/sympy,iamutkarshtiwari/sympy,ga7g08/sympy,bukzor/sympy,Titan-C/sympy,emon10005/sympy,sahmed95/sympy,bukzor/sympy,Designist/sympy,MridulS/sympy,skidzo/sympy,lindsayad/sympy,moble/sympy,skidzo/sympy,sunny94/temp,mcdaniel67/sympy,atsao72/sympy,jbbskinny/sympy,shipci/sympy,MechCoder/sympy,madan96/sympy,farhaanbukhsh/sympy,wyom/sympy,vipulroxx/sympy,jerli/sympy,pbrady/sympy,jbbskinny/sympy,Curious72/sympy,atreyv/sympy,saurabhjn76/sympy,Shaswat27/sympy,cccfran/sympy,Shaswat27/sympy,rahuldan/sympy,oliverlee/sympy,iamutkarshtiwari/sympy,sampadsaha5/sympy,ChristinaZografou/sympy,hargup/sympy,ChristinaZografou/sympy,vipulroxx/sympy,Titan-C/sympy,hargup/sympy,souravsingh/sympy,Sumith1896/sympy,shipci/sympy,yukoba/sympy,pandeyadarsh/sympy,yashsharan/sympy,saurabhjn76/sympy,farhaanbukhsh/sympy,kevalds51/sympy,Mitchkoens/sympy,sahilshekhawat/sympy,meghana1995/sympy,ga7g08/sympy,lidavidm/sympy,jaimahajan1997/sympy,ahhda/sympy,liangjiaxing/sympy,mafiya69/sympy,flacjacket/sympy,chaffra/sympy,meghana1995/sympy,Designist/sympy,Titan-C/sympy,MridulS/sympy,AunShiLord/sympy,beni55/sympy,asm666/sympy,sahilshekhawat/sympy,atsao72/sympy,Arafatk/sympy,kaichogami/sympy,emon10005/sympy,Gadal/sympy,Shaswat27/sympy,hargup/sympy,MridulS/sympy,atsao72/sympy,abhiii5459/sympy,maniteja123/sympy,cccfran/sympy,shikil/sympy,chaffra/sympy,vipulroxx/sympy,postvakje/sympy,kumarkrishna/sympy,oliverlee/sympy,liangjiaxing/sympy,lindsayad/sympy,ChristinaZografou/sympy,yukoba/sympy,dqnykamp/sympy,jerli/sympy,shipci/sympy,pandeyadarsh/sympy,amitjamadagni/sympy,shikil/sympy,asm666/sympy,moble/sympy,MechCoder/sympy,Vishluck/sympy,Vishluck/sympy,Designist/sympy,yashsharan/sympy,sampadsaha5/sympy,minrk/sympy,maniteja123/sympy,farhaanbukhsh/sympy,Gadal/sympy,toolforger/sympy,skirpichev/omg,VaibhavAgarwalVA/sympy,chaffra/sympy,beni55/sympy,postvakje/sympy,lidavidm/sympy,jaimahajan1997/sympy,mcdaniel67/sympy,ga7g08/sympy,drufat/sympy,toolforger/sympy,Davidjohnwilson/sympy,Sumith1896/sympy,kaichogami/sympy,sampadsaha5/sympy,jbbskinny/sympy,jerli/sympy,garvitr/sympy,meghana1995/sympy,sunny94/temp,lindsayad/sympy,abloomston/sympy,Davidjohnwilson/sympy,postvakje/sympy,AunShiLord/sympy,AkademieOlympia/sympy,bukzor/sympy,kaushik94/sympy,yukoba/sympy,moble/sympy,atreyv/sympy,pandeyadarsh/sympy,grevutiu-gabriel/sympy,garvitr/sympy,ahhda/sympy,VaibhavAgarwalVA/sympy,jaimahajan1997/sympy,toolforger/sympy,souravsingh/sympy,skidzo/sympy,jamesblunt/sympy,kumarkrishna/sympy,Vishluck/sympy,jamesblunt/sympy,wyom/sympy,debugger22/sympy,mcdaniel67/sympy,liangjiaxing/sympy,wanglongqi/sympy,srjoglekar246/sympy,jamesblunt/sympy,Arafatk/sympy,debugger22/sympy,madan96/sympy,garvitr/sympy,emon10005/sympy,kevalds51/sympy,pernici/sympy,MechCoder/sympy,lidavidm/sympy