Add documentation

docs/src/faq.md

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+# FAQ
+
+## Why is the nightly toolchain required?
+
+Building GRM currently requires nightly features due to the usage of
+[`std::path::Path::is_symlink()`](https://doc.rust-lang.org/std/fs/struct.FileType.html#method.is_symlink).
+See the [tracking issue](https://github.com/rust-lang/rust/issues/85748).
+
+`is_symlink()` is actually available in rustc 1.57, so it will be on stable in
+the near future. This would mean that GRM can be built using the stable toolchain!