add documentation

01-problem/README.md

@@ -0,0 +1,31 @@
+# The Problem
+
+We have a `CredentialManager` implementation which can be in two states: **locked** or **unlocked**.
+
+When the manager is locked, it can be unlocked with the `unlock(...)` method. Now unlocked, the `set(...)` and `get(...)` methods can be performed on the manager. Also, it can be locked again using the `lock()` method.
+
+```rust
+let mut pw = CredentialManager::new();
+pw.unlock("pssst");
+pw.set("foo", "bar");
+let val = pw.get("foo");
+pw.lock();
+```
+
+But with the current implementation, on a locked manager, the `get` and `set` could be performed which would result in erroneous results.
+
+```rust
+let mut pw = CredentialManager::new();
+pw.set("foo", "bar"); // -> 💥
+```
+
+Also, nothing prevents us from unlocking an already unlocked manager as well as locking an already locked manager.
+
+```rust
+let mut pw = CredentialManager::new();
+pw.unlock("pssst");
+pw.unlock("pssst"); // -> 💥
+
+let mut pw = CredentialManager::new();
+pw.lock(); // -> 💥
+```