add documentation

02-runtime-check/README.md

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+# Solution 1: Runtime Checks
+
+One solution to the problem would be to simply check the locked state and restrict methods to be only callable in the corrent locked state.
+
+```rust
+let mut pw = CredentialManager::new();
+pw.set("foo", "bar")?; // this would bail with an error result
+pw.unlock("pssst")?;
+pw.unlock("pssst")?; // this would bail with an error result
+pw.set("foo", "bar")?;
+let val = pw.get("foo")?;
+pw.lock()?;
+pw.lock()?; // this would bail with an error result
+```
+
+## Caveats
+
+Performing runtime checks on the state requires returning `Result` and also requires the check and handling of erroneous results on every call to the manager.