Add target os section to typeshare book

Author: darrell-roberts

docs/book.toml

@@ -1,5 +1,5 @@
 [book]
-authors = ["Jane Lewis", "Josh Rampersad"]
+authors = ["Jane Lewis", "Josh Rampersad", "Darrell Roberts"]
 language = "en"
 multilingual = false
 src = "src"

docs/src/SUMMARY.md

@@ -5,4 +5,5 @@
 - [Usage](./usage/usage.md)
     - [Annotations](./usage/annotations.md)
     - [Configuration](./usage/configuration.md)
+    - [Target OS](./usage/target_os.md)
 - [Contributing](./contributing.md)

docs/src/usage/target_os.md

@@ -0,0 +1,87 @@
+# Target OS
+
+The `--target-os` argument is an optional command line argument that allows you to specify a list of comma separated `target_os` values. In your
+Rust source code you can use the [`target_os`](https://doc.rust-lang.org/reference/conditional-compilation.html#target_os) attribute
+to restrict a type, variant or fields.
+
+If you do not use `--target-os` then typeshare will generate all types, variants and fields that are typeshared.
+
+## Example
+```
+./typeshare ./my_rust_project \
+  --lang=typescript \
+  --output-file=my_typescript_definitions.ts \
+  --target-os=linux,macos
+```
+
+The example above is stating any types, variants, fields that are typeshared and are not applicable for `linux` or `macos` will be omitted from
+typeshare type generation.
+
+## Supported `target_os` definitions.
+
+### Simple standalone.
+
+```rust
+#[cfg(target_os = "android")]
+pub struct MyType;
+```
+
+This type will only be generated if `--target-os` has `android` in the list of target_os values.
+
+### Simple not rule
+
+```rust
+#[cfg(not(target_os = "android"))]
+pub struct MyType;
+```
+This type will only be generated if `--target-os` does not include `android` in the list of target_os values.
+
+### Multiple not any rule
+
+```rust
+#[cfg(not(any(target_os = "android", target_os = "ios")))]
+pub struct MyType;
+```
+
+This type will only be generated if `--target-os` does not include `android` or `ios` in the list of target_os values.
+
+The following example will allow `MyType` to be typeshared.
+```
+./typeshare ./my_rust_project \
+  --lang=typescript \
+  --output-file=my_typescript_definitions.ts \
+  --target-os=linux,macos
+```
+
+The following example will not allow `MyType` to be typeshared.
+```
+./typeshare ./my_rust_project \
+  --lang=typescript \
+  --output-file=my_typescript_definitions.ts \
+  --target-os=android,macos
+```
+
+## Combined with features or other cfg attributes
+
+Typehsare will not take into consideration any other `cfg` attributes other than `target_os` when generating types.
+
+For example:
+
+```rust
+#[cfg(any(target_os = "android", feature = "android-test")]
+pub struct MyType;
+```
+
+```rust
+#[cfg(all(target_os = "android", feature = "android-test")]
+pub struct MyType;
+```
+
+```
+./typeshare ./my_rust_project \
+  --lang=typescript \
+  --output-file=my_typescript_definitions.ts \
+  --target-os=android
+```
+
+In both examples above, `MyType` will be typeshared.