README: add missing dependencies

The sources of most components are downloaded using git, which needs to

be installed. The sources of webrtc are downloaded using gclient, which

requires GTK+ 2.0 development files to be installed.

requires GTK+ 2.0 development files and curl to be installed.

You also need a few perl modules installed:

- YAML::XS

- File::Temp

- File::Slurp

- File::Path

- File::Copy::Recursive

- String::ShellQuote

- Sort::Versions

- Digest::SHA

                  libio-captureoutput-perl libfile-slurp-perl \

                  libstring-shellquote-perl libsort-versions-perl \

                  libdigest-sha-perl libdata-uuid-perl libdata-dump-perl \

                  git libgtk2.0-dev

                  libfile-copy-recursive-perl git libgtk2.0-dev curl

Starting a build