Maintainers.txt: Update Package Maintainer description
Maintainers.txt: Update Package Maintainer description

Add note that the Package Maintainer is responsible for reviewing and
pushing package changes to source control.

Cc: Andrew Fish <be8d29debaaf5173b4b7ecb4ace852732e71f7b9@apple.com>
Cc: Leif Lindholm <a613079f0455438b29e28a9c5b9eb3779a81a33a@linaro.org>
Cc: Michael D Kinney <fd20bc543a9f65bb633fac7d08a403c1f1c8eb6c@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>
Reviewed-by: Leif Lindholm <a613079f0455438b29e28a9c5b9eb3779a81a33a@linaro.org>
Reviewed-by: Michael D Kinney <fd20bc543a9f65bb633fac7d08a403c1f1c8eb6c@intel.com>
Reviewed-by: Andrew Fish <be8d29debaaf5173b4b7ecb4ace852732e71f7b9@apple.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2