version-control/README.md: Add a motivational paragraph

The text is adapted from 3976755 (Added the file for lesson 2,
2012-02-04), which I cherry picked out of boot-camps (where it is
d48ceea) [1].

[1] https://github.com/swcarpentry/boot-camps

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++Version control is a way to keep a track of changing files and store a
++history of those changes and the motivation behind them.  This
++organized history allows many people in a collaboration to make
++changes concurrently, while staying up to date with changes made by
++their collaborators.  It also makes it easy to keep yourself organized
++if you store similar files in several different locations (e.g. a
++project that you develop on both your home computer and your work
++computer).
++
 +Basic use
 +=========
 +
 +Learning objectives
 +-------------------
 +
 +* Draw a diagram showing the places version control stores
 +  information.
 +* Check out a working copy of a repository.
 +* View the history of changes to a project.
 +* Add files to a project.
 +* Commit changes made to the working copy.
 +* Get changes from an upstream repository.
 +* Compare the current state of a working copy to the most recent
 +  update from the upstream repository.
 +* Explain what "version 123 of `xyz.txt`" actually means.
 +
 +Key points
 +----------
 +
 +* Version control is a better way to manage shared files than email or
 +  shared folders.
 +* People share their changes by committing them to public repositories
 +  and fetch other's changes by updating their local copy from public
 +  repositories.
 +* The version control system prevents people from overwriting each
 +  other's work by making merges explicit.
 +* It also keeps a complete history of changes made to the master so
 +  that old versions can be recovered reliably.
 +* Version control systems work best with text files, but can also
 +  handle binary files such as images and Word documents.
 +* Every repository is identified by a URL.
 +* Each change to the master copy is identified by a unique revision
 +  ID.
 +* Revisions identify snapshots of the entire repository, not changes
 +  to individual files.
 +* Each change should be commented to make the history more readable.
 +* Commits are transactions: either all changes are successfully
 +  committed, or none are.
 +
 +Merging conflicts
 +=================
 +
 +Learning objectives
 +-------------------
 +
 +* Explain what causes conflicts to occur and how to tell when one has
 +  occurred.
 +* Resolve a conflict.
 +
 +Key points
 +----------
 +
 +* Conflicts must be resolved before a commit can be completed.
 +* Most version control systems put markers in text files to show
 +  regions of conflict.
 +
 +Recovering old versions
 +=======================
 +
 +Learning objectives
 +-------------------
 +
 +* Discard changes made to a working copy.
 +* Recover an old version of a file.
 +* Explain what branches are and when they are used.
 +
 +Key points
 +----------
 +
 +* Recovering an old version of a file may not erase the intervening
 +  changes.
 +* Use branches to support parallel independent development.
 +
 +Setting up a repository
 +=======================
 +
 +Learning objectives
 +-------------------
 +
 +* How to create a repository.
 +
 +Key points
 +----------
 +
 +* Repositories can be hosted locally, on local (departmental) servers,
 +  on hosting services, or on their owners' own domains.
 +
 +Provenance
 +==========
 +
 +Learning objectives
 +-------------------
 +
 +* What data provenance is.
 +* How to embed version numbers and other information in files managed
 +  by version control.
 +* How to record version information about a program in its output.
 +
 +Key points
 +----------
 +
 +* Put version numbers in programs' output to establish provenance for
 +  data.