Documentation updates (mostly about watermark strength).

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

README.adoc

@@ -51,6 +51,36 @@ and can be used for the add/get commands as follows:
   audiowmark add --key test.key in.wav out.wav 0123456789abcdef0011223344556677
   audiowmark get --key test.key out.wav
 
+== Watermark Strength
+
+The watermark strength parameter affects how much the watermarking algorithm
+modifies the input signal. A stronger watermark is more audible, but also more
+robust against modifications. The default strength is 10. A watermark with that
+strength is recoverable after mp3/ogg encoding with 128kbit/s. In our informal
+listening tests, this setting also has a very good subjective quality.
+
+A higher strength (for instance 15) would be helpful for instance if robustness
+against multiple conversions or conversions to low bit rates (i.e. 64kbit/s) is
+desired.
+
+A lower strength (for instance 6) makes the watermark less audible, but also
+less robust. Strengths below 5 are not recommended. To set the strength, the
+same value has to be passed during both, generation and retrieving the
+watermark. Fractional strengths (like 7.5) are possible.
+
+  audiowmark add --strength 15 in.wav out.wav 0123456789abcdef0011223344556677
+  audiowmark get --strength 15 out.wav
+
+Since the watermark modifies the input signal, it will make it louder at some
+positions. To avoid clipping, the input signal is scaled down during
+watermarking. For the strength values mentioned here everything should work,
+however for very strong watermarks you are likely to see a warning like this
+
+  audiowmark: warning: clipping occured in watermarked audio signal
+
+which can be fixed by modifying the scaling factor (which defaults to
+0.95) using `--pre-scale 0.9`.
+
 == Dependencies
 
 If you compile from source, audiowmark needs the follwing libraries:
@@ -59,6 +89,7 @@ If you compile from source, audiowmark needs the follwing libraries:
 * libsndfile
 * libgcrypt
 * libzita-resampler
+* libmpg123
 
 == Building fftw