Maintenance Fixes

[minin43]

• Dunno why the README.md is being updated
• Fixes an error with nz_button where it would attempt to index a nil value
• Gmod updated ent:SetMaterial to accept only strings and not Materials, updated wall_buys to reflect this
• Added in functionality to display a secondary CarryItem's text if the primary's is no longer valid
• Updated door/sh_sync to send some door data (flags) as bools instead of strings - this was causing Buffer Overflow exceptions in clients where maps used too many prop-barricades for doors - Important
• Update sh_zombierags to manually check the zombie is safe to remove, as using SafeRemoveEntityDelayed was causing errors - I'm assuming when the ent was already null
• Update round/sh_sync to use net.WriteUInt instead of net.WriteEnt to send clients updates about player ready status