Add libwebp to contributing doc

[scotts]

Adds libwebp to the contributing doc. Previously, it was not there and if you followed the default instructions, the webp tests would fail.

Note that libwebp pulls in standard jpeg, so we end up having both jpeg and libjpeg-turbo installed. All tests pass in my Linux environment, though, so I assume we're using libjpeg-turbo at runtime. It's worth noting what we do for CI. On Linux, we install both much the same as what I'm putting in our contributing doc:

vision/packaging/pre_build_script.sh

Lines 36 to 37 in 7b283c2

	conda install libwebp -y
	conda install libjpeg-turbo -c pytorch

But on Mac, we do this dance:

vision/packaging/pre_build_script.sh

Lines 16 to 21 in 7b283c2

	conda install libpng libwebp -y
	# Installing webp also installs a non-turbo jpeg, so we uninstall jpeg stuff
	# before re-installing them
	conda uninstall libjpeg-turbo libjpeg -y
	conda install -y ffmpeg=4.2 -c pytorch
	conda install -y libjpeg-turbo -c pytorch

I bring this up because our Mac contributors might run into the above.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/9119

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❌ 1 New Failure, 3 Unrelated Failures

As of commit 6bb8f8b with merge base 7b283c2 ():

NEW FAILURE - The following job has failed:

• Docs / build / linux-job (gh)

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Tests / unittests-macos (3.10, macos-m1-stable) / macos-job (gh) (trunk failure)
  test/test_ops.py::TestDeformConv::test_aot_dispatch_dynamic__test_forward[33-False-mps]
• Tests / unittests-macos (3.12, macos-m1-stable) / macos-job (gh) (trunk failure)
  test/test_ops.py::TestDeformConv::test_aot_dispatch_dynamic__test_forward[33-False-mps]
• Tests / unittests-macos (3.9, macos-m1-stable) / macos-job (gh) (trunk failure)
  test/test_ops.py::TestDeformConv::test_aot_dispatch_dynamic__test_forward[33-False-mps]

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