Rework README and documentation

The README.md page is HUGE. Split it up.

Also document audio forwarding and improve installation instructions for
each platform and user documentation.

PR #3774 <https://github.com/Genymobile/scrcpy/pull/3774>

doc/tunnels.md

@@ -0,0 +1,123 @@
+# Tunnels
+
+Scrcpy is designed to mirror local Android devices. Tunnels allow to connect to
+a remote device (e.g. over the Internet).
+
+To connect to a remote device, it is possible to connect a local `adb` client to
+a remote `adb` server (provided they use the same version of the _adb_
+protocol).
+
+
+## Remote ADB server
+
+To connect to a remote _adb server_, make the server listen on all interfaces:
+
+```bash
+adb kill-server
+adb -a nodaemon server start
+# keep this open
+```
+
+**Warning: all communications between clients and the _adb server_ are
+unencrypted.**
+
+Suppose that this server is accessible at 192.168.1.2. Then, from another
+terminal, run `scrcpy`:
+
+```bash
+# in bash
+export ADB_SERVER_SOCKET=tcp:192.168.1.2:5037
+scrcpy --tunnel-host=192.168.1.2
+```
+
+```cmd
+:: in cmd
+set ADB_SERVER_SOCKET=tcp:192.168.1.2:5037
+scrcpy --tunnel-host=192.168.1.2
+```
+
+```powershell
+# in PowerShell
+$env:ADB_SERVER_SOCKET = 'tcp:192.168.1.2:5037'
+scrcpy --tunnel-host=192.168.1.2
+```
+
+By default, `scrcpy` uses the local port used for `adb forward` tunnel
+establishment (typically `27183`, see `--port`). It is also possible to force a
+different tunnel port (it may be useful in more complex situations, when more
+redirections are involved):
+
+```
+scrcpy --tunnel-port=1234
+```
+
+
+## SSH tunnel
+
+To communicate with a remote _adb server_ securely, it is preferable to use an
+SSH tunnel.
+
+First, make sure the _adb server_ is running on the remote computer:
+
+```bash
+adb start-server
+```
+
+Then, establish an SSH tunnel:
+
+```bash
+# local  5038 --> remote  5037
+# local 27183 <-- remote 27183
+ssh -CN -L5038:localhost:5037 -R27183:localhost:27183 your_remote_computer
+# keep this open
+```
+
+From another terminal, run `scrcpy`:
+
+```bash
+# in bash
+export ADB_SERVER_SOCKET=tcp:localhost:5038
+scrcpy
+```
+
+```cmd
+:: in cmd
+set ADB_SERVER_SOCKET=tcp:localhost:5038
+scrcpy
+```
+
+```powershell
+# in PowerShell
+$env:ADB_SERVER_SOCKET = 'tcp:localhost:5038'
+scrcpy
+```
+
+To avoid enabling remote port forwarding, you could force a forward connection
+instead (notice the `-L` instead of `-R`):
+
+```bash
+# local  5038 --> remote  5037
+# local 27183 --> remote 27183
+ssh -CN -L5038:localhost:5037 -L27183:localhost:27183 your_remote_computer
+# keep this open
+```
+
+From another terminal, run `scrcpy`:
+
+```bash
+# in bash
+export ADB_SERVER_SOCKET=tcp:localhost:5038
+scrcpy --force-adb-forward
+```
+
+```cmd
+:: in cmd
+set ADB_SERVER_SOCKET=tcp:localhost:5038
+scrcpy --force-adb-forward
+```
+
+```powershell
+# in PowerShell
+$env:ADB_SERVER_SOCKET = 'tcp:localhost:5038'
+scrcpy --force-adb-forward
+```