Create sim primer

[arouinfar]

Marking as on-hold until the following primers are published:

• [ ] https://github.com/phetsims/circuit-construction-kit-dc/issues/167
• [ ] https://github.com/phetsims/circuit-construction-kit-dc-virtual-lab/issues/10
• [x] https://github.com/phetsims/projectile-motion/issues/147

[arouinfar]

• [x] Draft script - 3/13/18
• [x] Record rough version for script verification - 3/18/18
• [x] Review script verification draft, and adjust script as necessary - 3/22/18
• [x] Record another draft - 4/5/18
• [x] Review 2nd draft - 4/6/18
• [x] Produce primer and upload - 4/11/18
• [x] Caption primer - 4/11/18

[arouinfar]

Primer has been published. Closing.