Add informations/refactor update page

[iTrooz]

When I discovered the update process of the AppImages, I was confused about a few things

I modified the documentation to include information about these things, in the hope that new users will understand more clearly the update process and how to implement it right

Here is the list of commits with my justifications for making them :

a0c63b4 : My workflow initially used mv to rename the appimage after its creation, which caused the update to fail. I figured it out by looking at the binary data of the .zsync file, to see that the original filename was included in it, and that I needed to rename the appimage from the packaging tool directly

e52efab : I didn't personally have this problem, but I can see people confusing appimagetool/linuxdeploy/etc... with these "external tools" you talk about (which are AppImageUpdate/etc...). I don't know if I phrased it the best way possible, but it seems more understandable from my PoV

09662d1 : appimage-builder not being in the list might confuse people (it did for me at first) and they may think that appimage-builder is yet another tool to do yet another job. Since you already talk about other projects such as electron-builder, I thought appimage-builder would also fit in

c4df8dd : Addition to a0c63b4 to tell packagers how they should rename the AppImage if they want to

d1356a2 : TheAssassin explained me this on #appimage. I thought it it had its place in the documentation

043c99d : the travis links seems to be 404 now, so I changed them to link to the github

37725cc : All this information (that was here before + what I added) do not seem to be related to updates with external tools, so I moved it to its own paragraph before that

Feel free to disagree with any change I made, I'm open to discussion !

(I know that, taken one by one, these things aren't serious problems, but together they may cause some headache understanding the update process, and well, this project is about making packaging easy, right ?)

[TheAssassin]

Not sure how much experience you have with git, but normally, you should use the commit messages to discuss reasoning, the pros and cons etc. of your changes. It's okay for now, but next time you should put them in there.

Will have a look soon.