[#49] Clarify UnityFileSystemApi section in README

Lead with the fact that the bundled library normally needs no action,
state that it is backward but not forward compatible, fix the copy
destination path (UnityFileSystem/ at the repo root, not
UnityDataTool/UnityFileSystem/), and hyperlink the library files.

Author: SkowronskiAndrew

README.md

@@ -40,17 +40,15 @@ Refer to the [commit history](https://github.com/Unity-Technologies/UnityDataToo
 
 ## Getting UnityFileSystemApi
 
-UnityFileSystemApi is distributed in the Tools folder of the Unity Editor (from version 2022.1.0a14). The UnityDataTools repository includes a Windows, Mac, and Linux copy of the library in the `UnityFileSystem/` directory. 
+UnityDataTool uses the native `UnityFileSystemApi` library to read Unity Archives and SerializedFiles. **Normally you don't need to do anything with this library.** The repository already includes a recent Windows, Mac, and Linux copy in the [`UnityFileSystem/`](https://github.com/Unity-Technologies/UnityDataTools/tree/main/UnityFileSystem) directory, and using that bundled copy is the recommended way to run the tool.
 
-The library is backward compatible and can read data files from most Unity versions, so typically the version that is provided with UnityDataTools can be used "as is".
+The library is backward compatible but not forward compatible: a given version can read content from the same or older Unity versions, but may be unable to read content produced by a newer Unity Editor than the library itself. The bundled copy is updated periodically as Unity evolves, so in practice it can read content from just about any Unity version.
 
-To analyze data using the library from a specific version of the Unity Editor, copy the appropriate UnityFileSystemApi file from your Unity Editor installation (`{UnityEditor}/Data/Tools/`) to `UnityDataTool/UnityFileSystem/` prior to building:
+`UnityFileSystemApi` is also distributed in the `Data/Tools/` folder of the Unity Editor (for all versions since 2022.1.0a14). In the rare case that you need to read content from a Unity version newer than the bundled library, copy the matching file from your Unity Editor installation (`{UnityEditor}/Data/Tools/`) into the `UnityFileSystem/` directory before building:
 
-The file name is as follows:
-
-- Windows: `UnityFileSystemApi.dll`
-- Mac: `UnityFileSystemApi.dylib`
-- Linux: `UnityFileSystemApi.so`
+- Windows: [`UnityFileSystemApi.dll`](https://github.com/Unity-Technologies/UnityDataTools/blob/main/UnityFileSystem/UnityFileSystemApi.dll)
+- Mac: [`UnityFileSystemApi.dylib`](https://github.com/Unity-Technologies/UnityDataTools/blob/main/UnityFileSystem/UnityFileSystemApi.dylib)
+- Linux: [`UnityFileSystemApi.so`](https://github.com/Unity-Technologies/UnityDataTools/blob/main/UnityFileSystem/UnityFileSystemApi.so)
 
 ## How to Build