Add install troubleshooting tip about longpaths error on Windows (#1306)

Add a new install troubleshooting tips about filename being too long that happens on Windows (even when enabled at OS level)
Git default config issue

Author: Askelhardd

README.md

@@ -98,6 +98,7 @@ python -m xformers.info
 * the version of GCC that you're using matches the current NVCC capabilities
 * the `TORCH_CUDA_ARCH_LIST` env variable is set to the architectures that you want to support. A suggested setup (slow to build but comprehensive) is `export TORCH_CUDA_ARCH_LIST="6.0;6.1;6.2;7.0;7.2;7.5;8.0;8.6"`
 * If the build from source OOMs, it's possible to reduce the parallelism of ninja with `MAX_JOBS` (eg `MAX_JOBS=2`)
+* If getting error message `Filename longer than 260 characters` on Windows, make sure long paths are enabled at OS level, and also execute the command `git config --global core.longpaths true`
 
 
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