Add note about gas costs (#2079)

* Add note about gas costs

* Update wording

README.md

@@ -40,7 +40,7 @@ contract MyNFT is ERC721Full, ERC721Mintable {
 
 _If you're new to smart contract development, head to [Developing Smart Contracts](https://docs.openzeppelin.com/learn/developing-smart-contracts) to learn about creating a new project and compiling your contracts._
 
-To keep your system secure, you should **always** use the installed code as-is, and neither copy-paste it from online sources, nor modify it yourself.
+To keep your system secure, you should **always** use the installed code as-is, and neither copy-paste it from online sources, nor modify it yourself. The library is designed so that only the contracts and functions you use are deployed, so you don't need to worry about it needlessly increasing gas costs.
 
 ## Learn More
 

docs/modules/ROOT/pages/index.adoc

@@ -39,7 +39,7 @@ contract MyNFT is ERC721Full, ERC721Mintable {
 
 TIP: If you're new to smart contract development, head to xref:learn::developing-smart-contracts.adoc[Developing Smart Contracts] to learn about creating a new project and compiling your contracts.
 
-To keep your system secure, you should **always** use the installed code as-is, and neither copy-paste it from online sources, nor modify it yourself.
+To keep your system secure, you should **always** use the installed code as-is, and neither copy-paste it from online sources, nor modify it yourself. The library is designed so that only the contracts and functions you use are deployed, so you don't need to worry about it needlessly increasing gas costs.
 
 [[next-steps]]
 == Learn More