Add va_math.txt based on the README from the va_math submission.

va_math.txt

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+
+The following is from the README.va_math that was included with the
+initial contribution of the va_math module. I've removed the parts
+that are obviously not applicable, i.e. how to compile the library, to
+this bundled version of the library.
+
+--------
+License.
+--------
+
+  Verilog-A math library built for Icarus Verilog
+  http://www.icarus.com/eda/verilog/
+
+  Copyright (C) 2007-2008  Cary R. (cygcary@yahoo.com)
+
+  This program is free software; you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation; either version 2 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License along
+  with this program; if not, write to the Free Software Foundation, Inc.,
+  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+
+------------------------------------------
+Standard Verilog-A Mathematical Functions.
+------------------------------------------
+
+The va_math VPI module implements all the standard math functions provided
+by Verilog-A as Verilog-D system functions. The names are the same except
+like all Verilog-D system functions the name must be prefixed with a '$'.
+For reference the functions are:
+
+  $ln(x)       --  Natural logarithm
+  $log(x)      --  Decimal logarithm
+  $exp(x)      --  Exponential
+  $sqrt(x)     --  Square root
+  $min(x,y)    --  Minimum
+  $max(x,y)    --  Maximum
+  $abs(x)      --  Absolute value
+  $floor(x)    --  Floor
+  $ceil(x)     --  Ceiling
+  $pow(x,y)    --  Power (x**y)
+  $sin(x)      --  Sine
+  $cos(x)      --  Cosine
+  $tan(x)      --  Tangent
+  $asin(x)     --  Arc-sine
+  $acos(x)     --  Arc-cosine
+  $atan(x)     --  Arc-tangent
+  $atan2(y,x)  --  Arc-tangent of y/x
+  $hypot(x,y)  --  Hypotenuse (sqrt(x**2 + y**2))
+  $sinh(x)     --  Hyperbolic sine
+  $cosh(x)     --  Hyperbolic cosine
+  $tanh(x)     --  Hyperbolic tangent
+  $asinh(x)    --  Arc-hyperbolic sine
+  $acosh(x)    --  Arc-hyperbolic cosine
+  $atanh(x)    --  Arc-hyperbolic tangent
+
+The only limit placed on the x and y arguments by the library is that they
+must be numbers (not constant strings). The underlying C library controls
+any other limits placed on the arguments. Most libraries return +-Inf or
+NaN for results that cannot be represented with real numbers. All functions
+return a real result.
+
+
+------------------------------------------
+Standard Verilog-A Mathematical Constants.
+------------------------------------------
+
+The Verilog-A mathematical constants can be accessed by including the
+"constants.vams" header file. It is located in the standard include
+directory. Recent version of Icarus Verilog (0.9.devel) automatically
+add this directory to the end of the list used to find include files.
+For reference the mathematical constants are:
+
+  `M_PI        --  Pi
+  `M_TWO_PI    --  2*Pi
+  `M_PI_2      --  Pi/2
+  `M_PI_4      --  Pi/4
+  `M_1_PI      --  1/Pi
+  `M_2_PI      --  2/Pi
+  `M_2_SQRTPI  --  2/sqrt(Pi)
+  `M_E         --  e
+  `M_LOG2E     --  log base 2 of e
+  `M_LOG10E    --  log base 10 of e
+  `M_LN2       --  log base e of 2
+  `M_LN10      --  log base e of 10
+  `M_SQRT2     --  sqrt(2)
+  `M_SQRT1_2   --  1/sqrt(2)
+
+
+------------------
+Using the Library.
+------------------
+
+Just add "-m va_math" to your iverilog command line/command file and
+`include the "constants.vams" file as needed.
+
+------
+Thanks
+------
+
+I would like to thank Larry Doolittle for his suggestions and
+Stephen Williams for developing Icarus Verilog.
+
+
+--------
+The End.
+--------