Readme for existing parflow estimator.

[ian-bertolacci]

Need readme for existing parflow tool (https://github.com/ian-bertolacci/ParflowSizeTimePrediction).

Deliverables:

• Readme markdown document.

Done when:

• Readme documents current tool's functionality and behavior.
• Readme documents current tool's prediction module requirements

[ian-bertolacci]

Done when:

• Readme documents current tools usage.

[ian-bertolacci]

Related to #6

[ian-bertolacci]

Review notes:

• Simpler example (washita)
• convert readme example to bash script example (cat script.sh then run) (also do for sinusoidal)
• More general description of usage.