Update CONTRIBUTING.md

[davidbielik]

Description

The contributing steps didn't work for me since rimraf wasn't installed in my global npm modules.

To help future contributors, I would suggest added npm install to the contribution steps. Let me know if I'm wrong here!

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I ran into this error when running npm link after cloning:

776 verbose argv "/usr/local/bin/node" "/usr/local/bin/npm" "link"
777 verbose node v14.15.4
778 verbose npm  v7.18.1
779 error code 127
780 error path /Users/<user>/firebase-tools
781 error command failed
782 error command sh -c npm run clean && npm run build -- --build tsconfig.publish.json
783 error > firebase-tools@9.13.1 clean
783 error > rimraf lib dev
784 error sh: rimraf: command not found
785 verbose exit 127

Since rimraf wasn't found, I tried npm install first which installed the dependency needed to run npm link

Scenarios Tested

1. delete all global npm modules
2. try to follow new contributing steps
3. it should now work

Sample Commands

n/a

[google-cla]

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[davidbielik]

@googlebot I signed it!

[samtstern]

@davidbielik thank you!