[Cin] "Create a appimage" in manual

MatN has created a script to create an appimage of CinGG. This script improves the previous one (contained in .../cinelerra-5.1/blds) adding the possibility to get the appimage also for other architectures and with Context Help. This new bld_appimage.sh is contained in .../cinelerra-5.1 and also refers to the new directory .../cinelerra-5.1/tools. I am attaching the section of the manual that contains these changes, where you can learn more about how it works. Also in this section the contribution of MatN has been crucial. Phyllis if you want, you could delete the bld_appimage.sh contained in .../cinelerra-5.1/blds and leave only the new one. Or you can leave both.