Plugin upload causes "Received invalid response from the server." - mention proxy webserver

[Kagee]

Summary

Append to the readme "Plugin upload causes "Received invalid response from the server."" to mention that if you have Mattermost behind a reverse proxy (i would assume this is quite common), the allowed post size in the reverse proxy/webserver config must also be larger than com.github.matterpoll.matterpoll-x.y.z.tar.gz file. (currently ~60 MB)

[hanzei]

Thanks for the suggestion @Kagee 👍 Would you be open to updating the Troubleshooting section?

[kaakaa]

ref: mattermost/mattermost#26271