Create README.md (#19)

* Create README.md

* Add gif

* Small edits

README.md

@@ -0,0 +1,77 @@
+# Newton
+Newton is an n body gravity simulator. Given a start state describing a system, it will continuously calculate the gravitational forces exerted with the system and update the positions of each body.
+
+## Installation
+Newton is written entirely in `Rust`, therefore it can be easily built via `Cargo` and run directly.
+
+```
+git clone https://github.com/johnxnguyen/newton.git
+cd newton
+cargo build
+./target/debug/newton --help
+```
+
+## Usage
+Newton is a command line tool which requires a single argument: the path to a yaml file describing the initial state of a simulation. This configuration file contains information about how many bodies to construct, what values their properties have (mass, position, velocity), and how they group and relate together to form systems. Consider the following example:
+
+```YAML
+# rotating-disk.yaml
+
+gens: # This is a list of property generators.
+  -
+    # This generates masses between [0.1, 0.2]
+    name: mass
+    type: mass
+    min: 0.1
+    max: 0.2
+  -
+    # Generators have names, we will refer to these later.
+    name: trans
+    type: translation
+    x: {min: 200.0, max: 250.0}
+    y: {min: 0.0, max: 0.0}
+  -
+    # This generator produces the same velocity (0.0, 5.0) repeatedly.
+    name: vel
+    type: velocity
+    dx: {min: 0.0, max: 0.0}
+    dy: {min: 5.0, max: 5.0}
+  -
+    # This is the fourth and final type of generator.
+    name: rot
+    type: rotation
+    min: 0.0
+    max: 360.0
+
+
+bodies: # This is a list of body definitions.
+  -
+    name: stars     # Body definitions also have names.
+    num: 5000       # The number of bodies to create. Defaults to 1, if no number specified.
+    m: mass         # Mass. A concrete number, or the name of a gen.
+    t: trans        # Translation. A concrete value, or the name of a gen. Defaults to (0.0, 0.0).
+    v: vel          # Velocity. A concrete value, or the name of a gen. Defaults to (0.0, 0.0).
+    r: rot          # Rotation applied to t & v. A concrete value, or the name of a gen. Defaults to 0.0
+  -
+    name: black_hole
+    m: 6250.0
+
+
+systems: # This is a list of system definitions.
+  - name: stars       # This means that there is just one system, consisting of the disk
+  - name: black_hole  # called "stars" and the center called "black_hole".
+
+```
+
+We can then run this simulation by
+
+```
+./newton ./rotating-disk.yaml --output ./data --frames 300
+```
+
+This will run the simulation for 300 steps. For every step, a text file is produced containing the position of the bodies defined in `rotating-disk.yaml`. These files will be found in `./data`. You can then convert these text files into images (and then into a movie) using the tools of your choice. An easy way to do this is to use [Processing](https://processing.org). Here is the result.
+
+![](rotating-disk.gif)
+
+## More...
+If you're interested in the project and would like to know more about how it works, check out the wiki for an overview of the n body problem and its solution.