Add documentation for how to upload LSIF index to Sourcegraph

[olafurpg]

Our docs currently only cover the steps to generate the LSIF index.

[olafurpg]

Closing as wontfix. Uploading the index to Sourcegraph is the command src lsif upload

[kubukoz]

@olafurpg is there a particular reason why it couldn't be built into this project? It'd simplify the setup for e.g. github actions 😅