Add .idea/** into .gitignore for JetBrains IDE

[asdf2014]

We usually use PyCharm for reading the source code of Tensorflow, so it is necessary for most developers to add .idea/** into .gitignore for JetBrains IDE to ignore those non-project files.

[mention-bot]

@asdf2014, thanks for your PR! By analyzing the history of the files in this pull request, we identified @tensorflower-gardener, @benoitsteiner and @keveman to be potential reviewers.

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[asdf2014]

I signed it!

[googlebot]

CLAs look good, thanks!

[rmlarsen]

@tensorflow-jenkins test this please

[rmlarsen]

@tensorflow-jenkins test this please

[rmlarsen]

@tensorflow-jenkins test this please

[rmlarsen]

@tensorflow-jenkins test this please

[cancan101]

I think is generally not best practice to add IDE specific rules to a project gitignore. Rather have the developer use a global gitignore on his/her system. See for example: toptal/gitignore.io#4