Add info on citation of forester and metafor to CITATION

[mcguinlu]

As the new blobbogram/forest plot functionality in robvis depends heavily on the metafor and forester packages, include citation details for these and an explanation of the relationship in the CITATION file.

For example, from nasapower:

citHeader("While nasapower does not redistribute the data in any way,\n",
          "we encourage users to follow the requests of the POWER\n",
          "Project Team:\n",
          "\n",
          "'When POWER data products are used in a publication, we\n",
          "request the following acknowledgment be included:\n",
          "These data were obtained from the NASA Langley Research\n",
          "Center POWER Project funded through the NASA Earth Science\n",
          "Directorate Applied Science Program.'\n",
          "\n",
          "To cite nasapower in publications, please use:")