Moved contributing section from README to CONTRIBUTING

This is the preferred standard for git repository hosted on GitHub (see
https://github.com/blog/1184-contributing-guidelines)
Merge pull request #532 from dotta/wip/split-readme-contributing

Moved contributing section from README to CONTRIBUTING
Updated Tycho version to be compatible with both Maven 3.0.x and 3.1.x
Added a source bundle for the sbt library.

This commit allows debugging, hyperlinking, etc. towards
sbt sources. Useful when working on the Sbt builder.

* create a source bundle for the sbt library. I needed to
  inline the contents of sbt jars in order to get Eclipse
  to correctly associate sources with classfiles

* created a new compiler.interface bundle, only for the
  compiler interface, together with the matching
  `compiler.interface.sources` bundle. Sbt requires this to
  be a separate jar and there was no way to build a single sbt
  bundle with only some of the dependencies inlined.

A similar approach could be used for the Scala library/compiler
bundles, but we should investigate the possibility of having the
Scala team publish source bundles too (Re #1001889).

(this was originally a commit from @dragos, which I have adapted
based on the comments in https://github.com/scala-ide/scala-ide/pull/482)

Fixes #1001888
No need to put the sbt version number in the bundle name
Lifed up version for Sbt bundles in variable ${sbt.osgi.version}
No reason for putting the manifest in the ${basedir}

The one single reason for doing it is to allow to import in the workspace the
`sbt.full.library` project, so that it can be correctly consumed by the
dependent projects (e.g., the `core project). However, given we practically
never had to do so in the past 6 months, it feels reasonable to remove the
flexiblity in the name of a cleaner build. In case you will need to import he
`sbt.full.library` project in Eclipse at any point, just create the required
.classpath and .project, and copy/paste in the project's root the META-INF
folder generated by the build (look in the target/ folder). Make  sure to do
the same for the `sbt.compiler.interface` project.
Deleted unneeded import-package clauses in toolchain MANIFESTs

The maven build does no longer spits out warnings for unused package imports in MANIFESTs.
Adding organisation information in the POM to reduce duplication

In fact, the default value for Bundle-Vendor is ${pom.organization.name}