[cachetop] add example and man page.
make interval a positional parameter.
diff --git a/tools/cachetop_example.txt b/tools/cachetop_example.txt
new file mode 100644
index 0000000..95fd425
--- /dev/null
+++ b/tools/cachetop_example.txt
@@ -0,0 +1,70 @@
+# ./cachetop -h
+usage: cachetop.py [-h] [interval]
+
+show Linux page cache hit/miss statistics including read and write hit % per
+processes in a UI like top.
+
+positional arguments:
+ interval Interval between probes.
+
+optional arguments:
+ -h, --help show this help message and exit
+
+examples:
+ ./cachetop # run with default option of 5 seconds delay
+ ./cachetop 1 # print every second hit/miss stats
+
+# ./cachetop 5
+Buffers MB: 76 / Cached MB: 114
+PID UID CMD HITS MISSES DIRTIES READ_HIT% WRITE_HIT%
+ 1 root systemd 2 0 0 100.0% 0.0%
+ 680 root vminfo 3 4 2 14.3% 42.9%
+ 567 syslog rs:main Q:Reg 10 4 2 57.1% 21.4%
+ 986 root kworker/u2:2 10 2457 4 0.2% 99.5%
+ 988 root kworker/u2:2 10 9 4 31.6% 36.8%
+ 877 vagrant systemd 18 4 2 72.7% 13.6%
+ 983 root python 148 3 143 3.3% 1.3%
+ 981 root strace 419 3 143 65.4% 0.5%
+ 544 messageb dbus-daemon 455 371 454 0.1% 0.4%
+ 243 root jbd2/dm-0-8 457 371 454 0.4% 0.4%
+ 985 root (mount) 560 2457 4 18.4% 81.4%
+ 987 root systemd-udevd 566 9 4 97.7% 1.2%
+ 988 root systemd-cgroups 569 9 4 97.8% 1.2%
+ 986 root modprobe 578 9 4 97.8% 1.2%
+ 287 root systemd-journal 598 371 454 14.9% 0.3%
+ 985 root mount 692 2457 4 21.8% 78.0%
+ 984 vagrant find 9529 2457 4 79.5% 20.5%
+
+Above shows the run of `find /` on a newly booted system.
+
+Command used to generate the activity
+# find /
+
+Below shows the hit rate increases as we run find a second time and it gets it
+its pages from the cache.
+# ./cachetop.py
+Buffers MB: 76 / Cached MB: 115
+PID UID CMD HITS MISSES DIRTIES READ_HIT% WRITE_HIT%
+ 544 messageb dbus-daemon 2 2 1 25.0% 50.0%
+ 680 root vminfo 2 2 1 25.0% 50.0%
+ 243 root jbd2/dm-0-8 3 2 1 40.0% 40.0%
+ 1068 root python 5 0 0 100.0% 0.0%
+ 1071 vagrant bash 350 0 0 100.0% 0.0%
+ 1071 vagrant find 12959 0 0 100.0% 0.0%
+
+
+Below shows that the dirty pages increases as a file of 80M is created running
+# dd if=/dev/urandom of=/tmp/c bs=8192 count=10000
+
+# ./cachetop.py 10
+Buffers MB: 77 / Cached MB: 193
+PID UID CMD HITS MISSES DIRTIES READ_HIT% WRITE_HIT%
+ 544 messageb dbus-daemon 9 10 7 10.5% 15.8%
+ 680 root vminfo 9 10 7 10.5% 15.8%
+ 1109 root python 22 0 0 100.0% 0.0%
+ 243 root jbd2/dm-0-8 25 10 7 51.4% 8.6%
+ 1070 root kworker/u2:2 85 0 0 100.0% 0.0%
+ 1110 vagrant bash 366 0 0 100.0% 0.0%
+ 1110 vagrant dd 42183 40000 20000 27.0% 24.3%
+
+The file copied into page cache was named /tmp/c with a size of 81920000 (81920000/4096) = 20000