STYLE: Add detail to the vxl update instructions

Make the working directory for each command sequence explicit.  State
the purpose of the script recorded in the upstream snapshot branch
commit messages.  Add ascii-art to show the history shape as it is
constructed.  Add a Change-Id line to the upstream snapshot commits so
that the entire update topic can be reviewed in Gerrit.

Change-Id: I849330fe6bdd5c48f70c2f0a7916328936b3e9de
Merge topic 'vxl-update-instructions'

6f39a08 STYLE: Add detail to the vxl update instructions
ENH: Add double-conversion library for precise Float output

double-conversion is a library from (or at least hosted by) Google.

https://code.google.com/p/double-conversion/

Double-conversions advantage is that when it converts a floating
point number to a string, it uses the minimum length string representation
that when converted back into a floating point number, the new FP
number is bit-identical to the original.

Change-Id: I308d91a0f8ec6ca2d5d07c0a7b691b44c8fedc7c
Merge topic 'AddDoubleConversion'

8a2205a ENH: Add double-conversion library for precise Float output