Update documentation with adjustment to prevent loopback switching

source/sink

Today after a system update I noticed my denoised microphone source
wasn't outputting any sound.  This was a bit perplexing since I hadn't
recompiled or changed my pulse configuration.

I tried using the NoiseTorch project and found that it worked just fine,
so my pulse audio configuration seemed to be the cause. After a bit of
digging through pulseaudio's output, I found a line of interest:

`I: [pulseaudio] source.c: The source output 0 "(null)" is moving to
denoised due to change of the default source.`

I found that in my case source output 0 was the module-loopback.  The
source output changing sources broke the module chain and my actual
source was no longer being chained to the noise suppression plugin.

I found the `source_dont_move` and `sink_dont_move` options for the
loopback module here:
https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/Modules/#module-loopback.
These seem to be desirable options for this setup since we want the sink
and source in the loopback module to stay consistent.

Author: Mikhail Swift

README.md

@@ -39,7 +39,7 @@ Then, create the new device using:
 ```sh
 pacmd load-module module-null-sink sink_name=mic_denoised_out rate=48000
 pacmd load-module module-ladspa-sink sink_name=mic_raw_in sink_master=mic_denoised_out label=noise_suppressor_mono plugin=/path/to/librnnoise_ladspa.so control=50
-pacmd load-module module-loopback source=<your_mic_name> sink=mic_raw_in channels=1
+pacmd load-module module-loopback source=<your_mic_name> sink=mic_raw_in channels=1 source_dont_move=true sink_dont_move=true
 ```
 
 This needs to be executed every time PulseAudio is launched.
@@ -50,7 +50,7 @@ You can automate this by creating file in `~/.config/pulse/default.pa` with the
 
 load-module module-null-sink sink_name=mic_denoised_out rate=48000
 load-module module-ladspa-sink sink_name=mic_raw_in sink_master=mic_denoised_out label=noise_suppressor_mono plugin=/path/to/librnnoise_ladspa.so control=50
-load-module module-loopback source=your_mic_name sink=mic_raw_in channels=1
+load-module module-loopback source=your_mic_name sink=mic_raw_in channels=1 source_dont_move=true sink_dont_move=true
 
 set-default-source mic_denoised_out.monitor
 ```