Release notes for 2.6.0

Release notes for tclws 2.6.0.

Changes:

<table border=1>

<tr><td>Client Side</td><td>Use log::logsubst instead log::log to optimize non-log
<tr><td>Server Side</td><td>Use log::logsubst instead log::log to optimize non-log
<tr><td>Client Side</td><td>Fixed abstract type
</table>