Merge branch 'github-readme' into 'master'

Added GitHub README

See merge request !517

README.md

+SWIFT: SPH WIth Fine-grained inter-dependent Tasking
+====================================================
+
+[![Build Status](https://gitlab.cosma.dur.ac.uk/jenkins/job/GNU%20SWIFT%20build/badge/icon)](https://gitlab.cosma.dur.ac.uk/jenkins/job/GNU%20SWIFT%20build/)
+
+SWIFT is a gravity and SPH solver designed to run cosmological simulations
+on peta-scale machines, scaling well up to 10's of thousands of compute
+node.
+
+More general information about SWIFT is available on the project
+[webpages](http://www.swiftsim.com).
+
+For information on how to _run_ SWIFT, please consult the onboarding guide
+available [here](http://www.swiftsim.com/onboarding.pdf). This includes
+dependencies, and a few examples to get you going.
+
+We suggest that you use the latest release branch of SWIFT, rather than the
+current master branch as this will change rapidly. We do, however, like to
+ensure that the master branch will build and run.
+
+This GitHub repository is designed to be an issue tracker, and a space for
+the public to submit patches through pull requests. It is synchronised with
+the main development repository that is available on the
+[ICC](http://icc.dur.ac.uk)'s GitLab server which is available
+[here](https://gitlab.cosma.dur.ac.uk/swift/swiftsim).
+
+Please feel free to submit issues to this repository, or even pull
+requests. We will try to deal with them as soon as possible, but as the
+core development team is quite small this could take some time.
+
+Contribution Guidelines
+-----------------------
+
+The SWIFT source code uses a variation of the 'Google' formatting style.
+The script 'format.sh' in the root directory applies the clang-format-3.8
+tool with our style choices to all the SWIFT C source file. Please apply
+the formatting script to the files before submitting a pull request.
+
+Please check that the test suite still runs with your changes applied before
+submitting a pull request and add relevant unit tests probing the correctness
+of new modules. An example of how to add a test to the suite can be found by
+considering the tests/testGreeting case.
+
+Any contributions that fail any of the automated tests will not be accepted.
+Contributions that include tests of the proposed modules (or any current ones!)
+are highly encouraged.
+
+```
+ Welcome to the cosmological hydrodynamical code
+    ______       _________________
+   / ___/ |     / /  _/ ___/_  __/
+   \__ \| | /| / // // /_   / /   
+  ___/ /| |/ |/ // // __/  / /    
+ /____/ |__/|__/___/_/    /_/     
+ SPH With Inter-dependent Fine-grained Tasking
+
+ Website: www.swiftsim.com
+ Twitter: @SwiftSimulation
+
+See INSTALL.swift for install instructions.
+
+Usage: swift [OPTION]... PARAMFILE
+       swift_mpi [OPTION]... PARAMFILE
+
+Valid options are:
+  -a                Pin runners using processor affinity.
+  -c                Run with cosmological time integration.
+  -C                Run with cooling.
+  -d                Dry run. Read the parameter file, allocate memory but does not read
+                    the particles from ICs and exit before the start of time integration.
+                    Allows user to check validity of parameter and IC files as well as memory limits.
+  -D                Always drift all particles even the ones far from active particles. This emulates
+                    Gadget-[23] and GIZMO's default behaviours.
+  -e                Enable floating-point exceptions (debugging mode).
+  -f          {int} Overwrite the CPU frequency (Hz) to be used for time measurements.
+  -g                Run with an external gravitational potential.
+  -G                Run with self-gravity.
+  -M                Reconstruct the multipoles every time-step.
+  -n          {int} Execute a fixed number of time steps. When unset use the time_end parameter to stop.
+  -P  {sec:par:val} Set parameter value and overwrites values read from the parameters file. Can be used more than once.
+  -s                Run with hydrodynamics.
+  -S                Run with stars.
+  -t          {int} The number of threads to use on each MPI rank. Defaults to 1 if not specified.
+  -T                Print timers every time-step.
+  -v           [12] Increase the level of verbosity:
+                    1: MPI-rank 0 writes,
+                    2: All MPI-ranks write.
+  -y          {int} Time-step frequency at which task graphs are dumped.
+  -Y          {int} Time-step frequency at which threadpool tasks are dumped.
+  -h                Print this help message and exit.
+
+See the file examples/parameter_example.yml for an example of parameter file.
+```