Add a page about WASILibc module (#12)

* Add a page about `WASILibc` module

* Refine wording

* Refine wording

Author: MaxDesiatov

src/SUMMARY.md

@@ -9,6 +9,7 @@
     - [JavaScript interoperation](getting-started/javascript-interop.md)
     - [Swift Foundation](getting-started/foundation.md)
     - [Testing your app](getting-started/testing.md)
+    - [`WASILibc` module](getting-started/libc.md)
 - [Examples](examples/index.md)
     - [Importing function](examples/importing-function.md)
     - [Exporting function](examples/exporting-function.md)

src/getting-started/libc.md

@@ -0,0 +1,27 @@
+# `WASILibc` module
+
+When porting existing projects from other platforms to SwiftWasm you might stumble upon code that
+relies on importing a platform-specific [C
+library](https://en.wikipedia.org/wiki/C_standard_library) module directly. It looks like `import
+Glibc` on Linux, or `import Darwin` on Apple platforms. Fortunately, most of the standard C library
+APIs are available when using SwiftWasm, you just need to use `import WASILibc` to get access to it.
+Most probably you want to preserve compatibility with other platforms, thus your imports would look
+like this:
+
+```swift
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Glibc)
+import Glibc
+#elseif canImport(WASILibc)
+import WASILibc
+#endif
+```
+
+## Limitations
+
+WebAssembly and [WASI](https://wasi.dev/) provide a constrained environment, which currently does
+not directly support multi-threading, or filesystem access in the browser. Thus, you should not
+expect these APIs to work when importing `WASILibc`. Please be aware of these limitations when
+porting your code, which also has an impact on what [can be supported in the Swift
+Foundation](/getting-started/foundation.md) at the moment.