Add a README.md file for MJLib

MechJebLibTest/README.md

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+### MechJebLib
+
+A library of core astrodynamics functions rewritten and used by MechJeb.
+
+### Features
+
+* Gooding Lambert Solver
+* Shepperd's method of orbit propagation
+* Lots of reusable math functions
+* An IVP/ODE solver with DP5 and BS3 Runge Kutta implementations
+* A rewrite of PVG
+* A library of orbital maneuver solvers
+* A very permissive license
+
+### Conventions
+
+Unlike KSP there are numerous different conventions in MJLib.
+
+# Radians instead of Degrees
+
+The convention within Unity, KSP and MJ is to use degrees.  This code
+in MJLib breaks with this convention and uses radians exclusively.  It
+makes writing complex mathematical code substantially easier.
+
+# Right Handed Primitives
+
+The V3, M3 and Q3 primitives are double-precision right handed 3-vectors,
+3x3 matricies and quaternions.  This again makes dealing with complex
+mathematical algorithms substantially easier and avoids tracking down
+annoying handedness issues.  Vectors passed into MJLib should also be
+"derotated" and recentered so that they are in proper BCI coordinate
+systems.
+
+# No Linking to KSP or Unity
+
+This keeps the library fully self-contained and portable.
+
+* No use of the KSP Orbit class, everything is typically done with state
+  vectors and propagation by Shepperd's method.
+* No use of Debug.Log, the logger gets dependency injected by the
+  MechJebCore class
+* This keeps MJLib testable by avoiding the problem of linking against
+  KSP/Unity pulling in the Unityplayer and that getting into fights
+  with the test runner class.  KSP Mods have a difficult problem with
+  tests since they link against the shipped KSP binaries.
+
+There is an exception to this rule where the VesselBuilder/VesselUpdater
+used in the FuelFlowSimulation links to KSP/Unity, but there's also a
+comment there about how that needs to be moved to an auxiliary dll of
+glue code in between MJLib and KSP/Unity.  Generally it is more the
+responsibility of MJ itself to convert between KSP/Unity and MJLib.
+
+# NED (North, East, Down) and RPY conventions for orientation
+
+KSP used a crazy convention where the rocket standing on the pad has an
+'up' orientation which is the pointy end.  The 'forward' end is usually
+pointing across the water from KSC and down points south.  This is all
+due ultimately to Unity's left handed coordinate systems along with the
+choice to orient rockets like they are standing on the pad.
+
+The V3.forward/up/right vectors are very different from Unity and KSP and
+correspond to NED (which means that V3.up is actually 0,0,-1).  These
+correspond, though, to the axes of the RPY Euler angle rotations and are
+correct for dealing with spacecraft/aircraft.
+
+# Tests
+
+Tests are good, and there still aren't enough tests here.  I was able to
+completely rewrite the ODE integrator in PVG and ship it without firing
+up KSP and without breaking anyone because I had test coverage.
+