Add the LLVM dir to the .ignore file (#419)

LLVM is now a submodule and no longer in `.gitignore`. Thus, some dev
tools I use, like `rg` and `fd`, now consider the `llvm/` directory.
This PR fixes that by adding `llvm/` to the `.ignore` file.

Signed-off-by: xermicus <bigcyrill@hotmail.com>

.ignore

@@ -0,0 +1,2 @@
+# This is no longer in .gitignore; we add it here (honored by tools like rg and fd).
+llvm/