Update Readme/ wiki with prerequisite

Hi,

I'm giving this plugin a try.
I use jenkins 1.658, Debian Package Builder 1.6.11 on debian 7.10

Here's what I think it should be added:
- Update the readme/wiki by saying jenkins' user must have sudo rights to do

```
apt-get -y update
apt-get -y install aptitude pbuilder
```
- a gpg have to setup in jenkins global config even if on the job's config page "GPG sign package?" is unchecked. Here's what I got with this setup

```
00:00:08.707 [debian-package-builder] running command
00:00:08.708 [workspace] $ /bin/sh -xe /tmp/hudson4911813069838015475.sh
00:00:08.711 + gpg --list-key null
00:00:08.915 gpg: error reading key: clef publique introuvable
00:00:08.917 ERROR: Build step failed with exception
00:00:08.920 java.lang.NullPointerException
00:00:08.921 at java.io.Writer.write(Writer.java:157)
00:00:08.921 at hudson.FilePath$17.invoke(FilePath.java:1373)
00:00:08.922 at hudson.FilePath$17.invoke(FilePath.java:1356)
00:00:08.922 at hudson.FilePath.act(FilePath.java:990)
00:00:08.923 at hudson.FilePath.act(FilePath.java:968)
00:00:08.923 at hudson.FilePath.createTextTempFile(FilePath.java:1356)
00:00:08.924 at hudson.FilePath.createTextTempFile(FilePath.java:1329)
00:00:08.924 at ru.yandex.jenkins.plugins.debuilder.DebianPackageBuilder.importKeys(DebianPackageBuilder.java:319)
00:00:08.925 at ru.yandex.jenkins.plugins.debuilder.DebianPackageBuilder.perform(DebianPackageBuilder.java:101)
00:00:08.925 at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
00:00:08.926 at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:782)
00:00:08.926 at hudson.model.Build$BuildExecution.build(Build.java:205)
00:00:08.926 at hudson.model.Build$BuildExecution.doRun(Build.java:162)
00:00:08.927 at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:534)
00:00:08.927 at hudson.model.Run.execute(Run.java:1738)
00:00:08.928 at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
00:00:08.928 at hudson.model.ResourceController.execute(ResourceController.java:98)
00:00:08.928 at hudson.model.Executor.run(Executor.java:410)
00:00:08.929 Build step 'Build debian package' marked build as failure
```

Thanks for your work :D

Check the README or wiki for the installation and job-configuration prerequisites, then inspect ru.yandex.jenkins.plugins.debuilder.DebianPackageBuilder.java around importKeys at line 319 and perform at line 101. Document the required sudo access and GPG setup, and verify whether the reported failure needs a separate bug issue.

Written by the indexing model from the issue text.