Changelog fixed

In commit 606b65a4ab716fc4cef46a5ffd493509c4d09ec2
I wrote that error was due to user typing "make install"
without "make". I now realized that the error was always there
when PLUMED was installed. I modify the CHANGELOG to make
it more clear. I will also send a patch on the mailing list.
Merge branch 'v2.0' into v2.1