Update i.spec.sam.html (#986)

Added the description for the input matrix file containing the signatures (copied shamelessly from i.spec.unmix manual)

src/imagery/i.spec.sam/i.spec.sam.html

@@ -2,6 +2,21 @@ <h2>DESCRIPTION</h2>
 
 <p><em>i.spec.sam</em> is used to perform Spectral Angle Mapping.
 
+<p><em>Input matrix of signatures</em>
+# Channels: r g b i1 i2 i3
+# Enter spectra linewise!
+# 1. Sagebrush
+# 2. Saltbush
+# 3. Soil
+# 4. Dry grass
+#
+Matrix: 4 by 6
+row0:  8.87  13.14  11.71  35.85  28.26 10.54
+row1: 13.59  20.12  19.61  70.66 34.82 16.35
+row2: 28.26  34.82  38.27  40.1 38.27 23.7
+row3: 10.54  16.35  23.7   38.98 40.1 38.98
+
+
 <h2>EXAMPLES</h2>
 
 <div class="code"><pre>