readme: Add mention of allowing Github application use within organization

[paulirish]

We ran into an issue on Lighthouse which took a long time to debug. We were seeing "404: Not found" when bundlesize ran on travis.

Turns out Github sometimes returns a 404 for authentication errors (rather than 401). The fix was adding bundlesize to the approved applications list for the github organization.

That happens here: https://github.com/settings/connections/applications/6756cb03a8d6528aca5a

I believe this depends on the organization settings. I imagine we have some non-default permission settings inside the Org i ran into this with.

[reznord]

/cc @siddharthkp

[sreetamdas]

'dra

…

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[zeptonaut]

For what it's worth, this also tripped me up until I searched for "organization" within the Github issues and found this 😄