Patches for configure Script and Doc

[Max Barraclough]

A while ago I had trouble getting lightning to configure on Ubuntu with:

    $ ./configure --enable-disassembler

At the time I mistakenly thought the issue was with the configure script 
failing to detect binutils despite that I had installed Ubuntu's 'binutils-dev' 
package, [0] but the issue was that I hadn't installed the other necessary 
dependencies. The configure script's message was telling me it couldn't find 
binutils, but in fact it was libz and libiberty that were the trouble. (The 
configure script was correctly checking for the presence of these dependencies, 
the issue was just with the messages.)

This command is what's needed on Ubuntu (and probably also Debian) systems if 
you want to build with the disassembler:

    $ sudo apt-get install binutils-dev libiberty-dev zlib1g-dev

The README-hacking file didn't mention dependencies. I've included a patch to 
add a mention there, and another patch to mention dependencies in the 
documentation file.

I've also included a patch to improve the warning/error messages produced by 
the configure script if it fails to find one of those dependencies.

Lastly I've included a patch to overhaul the repo's .gitignore files.

(I'm new to email-driven git workflows so I hope they come through ok.)

   -Max


[0] https://lists.gnu.org/archive/html/lightning/2020-06/msg00005.html

0001-Improve-README-hacking-re.-building-lightning.patch
Description: 0001-Improve-README-hacking-re.-building-lightning.patch

0002-Update-documentation-file-to-mention-dependencies.patch
Description: 0002-Update-documentation-file-to-mention-dependencies.patch

0003-Fix-configure-script-s-handling-of-disassembler.patch
Description: 0003-Fix-configure-script-s-handling-of-disassembler.patch

0004-Overhaul-.gitignore-files.patch
Description: 0004-Overhaul-.gitignore-files.patch