Added a README for the checking accounts example

basics/checking-accounts/README.md

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+# Checking Accounts
+
+Solana Programs should perform checks on instructions to ensure security and that required invariants
+are not being violated.
+
+These checks vary and depend on the exact task of the Solana Program.
+
+In this example we see some of the common checks a Solana Program can perform:
+
+- checking the program ID from the instruction is the program ID of your program
+- checking that the order and number of accounts are correct
+- checking the initialization state of an account
+- etc.