. gitignore 文件，我应该把它放在我的 xcode 项目中的什么位置？

You should put the .gitignore file into the project root directory.

You can have a .gitignore in every single directory of your project.

However, the best practice is to have one single .gitignore file on the project root directory, and place all files that you want to ignore in it, like this:

ignoredFile.whatever
ignoredDirectory/*
directory/ignoredFileInsideDirectory
.svn

Once you create the .gitignore file, the ignore files that have changes or are new into the repository will not be shown to as waiting for commit.

You can also have one global local git ignore file, that will manage all of your git repositories.

git config --global core.excludesfile ~/.gitignore_global

This alternative is particularly interesting for ignored files that are non-project related, such as IDE related files and so on.

You should not put tool-specific files into a project's .gitignore file — unless that tool is required to be used by each developer of the project.

Instead, create your personal global .gitignore file in your home directory and activate it:

git config --global core.excludesfile ~/.gitignore

Now you can put whatever files are generated by your development tools into your .gitignore file.

where should i put the .gitignore file?, is it inside the .git folder?

There is no .gitignore inside a .git folder.
The is however a .git/info/exclude file, which allows you to exclude a file locally, generally when you want to exclude a file only for this specific repo (with an alias like this one):

git config alias.exclude "!sh -c 'echo \"$1\" >> .git/info/exclude' -"
git exclude UserInterfaceState.xcuserstate

If you need to exclude this file for all similar project, as Elnur Abdurrakhimov mentioned in his answer, exclude it globally.

echo UserInterfaceState.xcuserstate >> ~/.gitignore

But, as mentioned in "Do we need to check in *.xcuserstate?", if it is a type of file which should be ignored by any user of that repo, I would actually version a .gitignore at the top of the repo, with a content similar to:

*.xcuserstate

That way, any user of that repo won't even realize that this file exists and is generated (whatever its name is): it won't show up in a git status ever.

Or you can ignore xcuserdata/ entirely (either in a global .gitignore, or in a versioned one, like the generic GitHub Objective-C.gitignore file.

There are there approaches to ignore files in git:

1. Global git ignore: - In this approach your custom rules will be applied to every git directory in your machine. Usually this file might be created under ~/.gitignore_global path. This approach good if you want to exclude some type files which is generated by OS or any other logging, cashing and etc. tools.
2. Per-repository git ignore: - In this approach your custom rules will be applied to specific git directory in your machine. Local per-repository rules can be added to the .git/info/exclude file in your repository. The rules specified under this file will not be committed, which means it will not be shared with others. Generally this approach useful to prevent committing of locally generated files. For example, for files created by your editor.
3. Per path git ignore: - In this case your custom rules will be applied to specific git repositories sub path. You can create .gitignore in any sub path of your specific repository. This approach usually used to ignore some types of file only under specific folder of specific repository. While this files can be added to version controlling by other paths and other repositories.

So, My suggest to you is, if you want to ignore your specific file (UserInterfaceState.xcuserstate) in your all repositories the best approach is to use global git ignore approach. However if your file is generated only in specific repository you can use per-repository git ignore approach.

For more information you can read this great blog post. And there per language specific git ignore rules can be founded here. And you can auto generate general git ignore rules for specific language, tool or os using this web site.

typically, its best practice to include the .gitignore file in the project's root directory