Add some documentation

README.fakedata.md

+
+swiftmpifakestepsim
+===================
+
+The swiftmpifakestepsim program works much like swiftmpistepsim, except that
+it generates fake logs:
+
+```
+Usage: ./swiftmpifakestepsim [options] nr_messages logfile.dat
+ options: -v verbose, -d data check, -s size (bytes/scale), 
+         -f <1|2> randomize injection order, 1 == just sends, 2 == sends and recvs
+        [-r uniform random from 1 to size, | 
+        -r -g half gaussian random from 1 with 2.5 sigma size., | 
+        -r -c <file> use cdf from file, size is a scale factor., |
+        -r -o <file> use occurence sample of values in a file, size is a scale factor.,] 
+        -x random seed
+```
+
+To use a fixed message size just use `-s`.
+
+To use a uniform distribution in the range 1 to N:
+```
+   -s N -r
+```
+
+To use a half gaussian (so biased towards smaller packets) use:
+```
+   -r -s <scale> -g
+```
+
+The cdf option reads a simple text file with a sampling of a cumulative
+distribution function, where each line has three values, the minimum and
+maximum range of the current bin and the value. Note that the value column
+should be normalized into the range 0 to 1.
+```
+  -r -s <scale> -c <cdf_file>
+```
+
+The occurrence file has just one value per line, these should present the
+sizes of the packets, this is used to form a cdf:
+```
+  -r -s <scale> -o <occurrence_file>
+```
+
+Other options are useful to make sure that the randoms are different, `-x` and
+that they run in different order `-f <1|2>`.
+
+Peter W. Draper 24 Apr 2023
+---------------------------