gave smart_pointers its own README.md

Author: seporterfield

exercises/smart_pointers/README.md

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+## Smart Pointers
+In Rust, smart pointers are variables that contain an address in memory and reference some other data, but they also have additional metadata and capabilities.
+Smart pointers in Rust often own the data they point to, while references only borrow data.
+
+## Further Information
+
+- [Smart Pointers](https://doc.rust-lang.org/book/ch15-00-smart-pointers.html)
+- [Using Box to Point to Data on the Heap](https://doc.rust-lang.org/book/ch15-01-box.html)
+- [Rc\<T\>, the Reference Counted Smart Pointer](https://doc.rust-lang.org/book/ch15-04-rc.html)
+- [Shared-State Concurrency](https://doc.rust-lang.org/book/ch16-03-shared-state.html)
+- [Cow Documentation](https://doc.rust-lang.org/std/borrow/enum.Cow.html)