Implications of setting READ_Write
You can set the database to SINGLE_USER so that no other session can come in while you are performing your changes. Also, sp_addrolemember has been deprecated; you should be using ALTER ROLE instead. Finally, please get in the habit of using statement terminators.

USE master;
GO

ALTER DATABASE SQL SET SINGLE_USER WITH ROLLBACK IMMEDIATE;
ALTER DATABASE SQL SET READ_WRITE;
GO

USE SQL;
GO

ALTER ROLE db_datareader ADD MEMBER [USER];
GO

USE master;
GO

ALTER DATABASE SQL SET READ_ONLY;
ALTER DATABASE SQL SET MULTI_USER;
GO