add documentation

03-builder-pattern/README.md

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+# Solution 2: "Builder Pattern"
+
+A better solution is to use two different structs for each state with their separate implementations. That ensured on a compiler level that the manager can not be misused in either state it is.
+
+```rust
+let pw = LockedCredentialManager::new();
+pw.set("foo", "bar");
+```
+
+This produces the following compiler error:
+
+```
+error[E0599]: no method named `set` found for struct `LockedCredentialManager` in the current scope
+  --> 03-builder-pattern\src\main.rs:45:8
+   |
+ 5 | struct LockedCredentialManager {
+   | ------------------------------ method `set` not found for this struct
+...
+45 |     pw.set("foo", "bar");
+   |        ^^^ method not found in `LockedCredentialManager`
+```
+
+And because we pass the struct by reference in the `unlock` and `lock` methods, resulting from Rust's [onwership memory model](https://doc.rust-lang.org/book/ch04-01-what-is-ownership.html), the previous object can not be used after the transformation. And because this is checked at compile time, it makes the double locking and unlocking a non-issue in production. Let's see what the following code results in.
+
+```rust
+    let pw = LockedCredentialManager::new();
+    let mut pw = pw.unlock("pssst");
+    pw.lock();
+    pw.lock();
+```
+
+This code results in the following compiler error:
+
+```
+error[E0382]: use of moved value: `pw`
+  --> 03-builder-pattern\src\main.rs:49:5
+   |
+45 |     let mut pw = pw.unlock("pssst");
+   |         ------ move occurs because `pw` has type `UnlockedCredentialManager`, which does not implement the `Copy` trait
+...
+48 |     pw.lock();
+   |        ------ `pw` moved due to this method call
+49 |     pw.lock();
+   |     ^^ value used here after move
+   |
+note: `UnlockedCredentialManager::lock` takes ownership of the receiver `self`, which moves `pw`
+  --> 03-builder-pattern\src\main.rs:36:17
+   |
+36 |     pub fn lock(self) -> LockedCredentialManager {
+   |                 ^^^^
+```