A Short Introduction to PF: A C++ Library for Particle Filtering

Features added related to the Journal of Open Source Software review process:

• clearer README.md including more details on build options
• a division by zero error has been fixed that pops up when using on MSVC
• add seed-setting constructor to all resampler types
• added a CONTRIBUTING.md file
• added a student t random number generator
• added an "almost constant velocity model" example for comparing pf implementations to SMCTC implementations