Add example usage for mc-sgx-sync and std::sync

Add developer notes about `mc-sgx-sync` module hierarchy.

.markdownlint-cli2.jsonc

@@ -10,5 +10,5 @@
 
   // Define glob expressions to use (only valid at root)
   "globs": ["**/*.md"],
-  "ignores": [".github/pull_request_template.md"]
+  "ignores": [".github/pull_request_template.md", "target/"]
 }

sync/README.md

@@ -7,7 +7,95 @@
 -->[![Docs Status][docs-image]][docs-link]<!--
 -->[![Dependency Status][deps-image]][deps-link]
 
-Synchronization primitives for SGX enclaves
+Synchronization primitives for SGX enclaves.
+
+The available primitives are meant to mimic the behavior
+of [std::sync](https://doc.rust-lang.org/std/sync/). Only the primitives
+whose behavior can be supported in SGX enclaves are supported.
+
+## Examples
+
+To have code that works with both
+[std::sync](https://doc.rust-lang.org/std/sync/) and
+[mc-sgx-sync](https://docs.rs/mc-sgx-sync/latest/mc_sgx_sync/).
+
+```rust
+#[cfg(feature = "sgx")]
+use mc_sgx_sync::Mutex;
+#[cfg(not(feature = "sgx"))]
+use std::sync::Mutex;
+
+let mutex = Mutex::new(5);
+
+{
+    let mut data = lock.lock().unwrap();
+    *data += 1;
+    assert_eq!(*data, 6);
+} // lock is dropped here
+```
+
+## Developer Notes
+
+The modules are implemented to mimic the layout of
+[std::sync](https://doc.rust-lang.org/std/sync/).
+
+<!-- Direct image link was chosen due to cargo docs not processing mermaid
+     sections. Kept the graph in this comment for future maintainers
+```mermaid
+graph LR
+    subgraph mc-sgx-sync
+    subgraph Public API
+        M(mutex.rs)
+        C(condvar.rs)
+        R(rwlock.rs)
+    end
+    subgraph mc-sgx-sync::sys
+        M_(mutex.rs)
+        C_(condvar.rs)
+        R_(rwlock.rs)
+    end
+    end
+    subgraph mc-sgx-tstdc
+        M-(mutex.rs)
+        C-(condvar.rs)
+        R-(rwlock.rs)
+    end
+    M -> M_ -> M-
+    C -> C_ -> C-
+    R -> R_ -> R-
+```
+-->
+![module hierarchy](https://mermaid.ink/svg/pako:eNp1ks1qAyEQgF9lmdMWMn0AD4Wy7aHQhWCvwmJ0mixZNfjTJoS8e83akLTEueh8M_Ix6hGU0wQM1l7uNs07F7bJEdKqAKMwrPcYDlb9qyzTahpV87x8K4Vz9K1JkfaPPjxcYdcqZ_WX9H8xb_335NT2Ssnqup2xcAg3puGuaqi4hrqsao0hanVjxLtGrBixbuwbxKc8QVmwwG7OugK7X8jnjBfIERZgyBs56vxgx3OLgLghQwJY3mrptwKEPeW-tNMy0qseo_PAPuUUaAEyRfeR7xJY9IkuTS-jzHObC6T5TF9-xfw5Tj-XYKLq)
+
+- The modules that define the public API are more or less copies from the
+  [rust source](https://github.com/rust-lang/rust.git) at commit
+  [606c3907](https://github.com/rust-lang/rust/commit/606c3907251397a42e23d3e60de31be9d32525d5)
+  with unsupported behavior removed. This ensures that clients can jump back and
+  forth between the [std::sync](https://doc.rust-lang.org/std/sync/) types and
+  the supported [mc-sgx-sync](https://docs.rs/mc-sgx-sync/latest/mc_sgx_sync/)
+  types.
+
+- The `mc-sgx-sync::sys` modules mimic the modules in the
+  [rust source](https://github.com/rust-lang/rust.git). The `sys` modules in the
+  [rust source](https://github.com/rust-lang/rust.git) are per operating system
+  or platform.
+  [mc-sgx-sync](https://docs.rs/mc-sgx-sync/latest/mc_sgx_sync/) only supports
+  SGX enclaves, but maintaining the `sys` layout reduces modifications to the
+  public API modules.
+
+- [mc-sgx-tstdc](https://docs.rs/mc-sgx-tstdc/latest/mc_sgx_tstdc/) provides
+  rust wrappers around the C implementation of the synchronization primitives.
+
+[mc-sgx-sync](https://docs.rs/mc-sgx-sync/latest/mc_sgx_sync/) could depend on
+[mc-sgx-tstdc-sys](https://docs.rs/mc-sgx-tstdc-sys/latest/mc_sgx_tstdc_sys/)
+and call the C implementation directly. This is how many of the `sys` modules in
+the [rust source](https://github.com/rust-lang/rust.git) are implemented. The
+choice to depend on
+[mc-sgx-tstdc](https://docs.rs/mc-sgx-tstdc/latest/mc_sgx_tstdc/) was made to be
+consistent with the use of other `mc-sgx-<lib_wrapper>-sys` crates. The
+`mc-sgx-<lib_wrapper>` crates provides idiomatic rust interfaces over the C API
+and are usually the only crates that directly depend on the
+`mc-sgx-<lib_wrapper>-sys` crates.
 
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