Linux only: full system-wide install with uSockets + Hints for CMake users

[talksik]

Fixes: #1600

1. Ensures that uSockets is installed with the make install command.
2. Gives hints as to the nuances with the git submodule for uSockets.
3. Gives hints on how to use with CMake.

[talksik]

@uNetworkingAB any thoughts on this?

[talksik]

bump

[talksik]

Can someone with write access please approve this? It doesn't let me merge otherwise.

Picking an arbitrary author @gubaojian would you be able to help with this?

[uNetworkingAB]

The uSockets install lines could be added as an install target in uSockets itself. uWebSockets is a separate project that already installs itself. If anything, uSockets could get its own "make install".

The "how to clone a repo" and "CMake snippets" texts are irrelevant to this repo.

[laoshanxi]

simply, this works for your compile environment:

git clone --depth=1 https://github.com/uNetworking/uWebSockets.git
cd uWebSockets
git submodule update --init --recursive --depth=1
make && make install
cp uSockets/src/libusockets.h /usr/local/include/
cp uSockets/uSockets.a /usr/local/lib/libuSockets.a