Add note for network buffers (Linux) in README

[ethaniccc]

This PR adds a note to the README notifying the user about increasing the limit of network buffers if they are on Linux.

Context: During testing on Zeqa, using the Spectral transport, the proxy after a period of time would be unable to read or write packets to the downstream server. Testing Spectrum TCP transport, a similar issue occurred, where after ~60 players, the proxy would start disconnecting players because of snappy: corrupt input With the help of @AkmalFairuz, it was determined that the network buffer limits were causing these issues. After increasing the network buffer size, the proxy was able to manage with 130 players with no errors or disconnects