Add PLEP index (with reserved numbers) and template

[namurphy]

This pull request adds PLEP 0 as an index of PLEPs.

This also reserves the following PLEP numbers for PlasmaPy organizational stuff:

number	title	author(s)	type
1	Purpose and Guidelines of PLEPs		informational
2	PlasmaPy Organizational Structure		process
3	Coordinating Committee Members		informational
4	PlasmaPy License		process
5	Versioning and Releases		process

• We undoubtedly need PLEP 1 which will include the process for how PLEPs are created and decided upon.
• We may need an organizational structure PLEP, which I tentatively put as PLEP 2. An alternative would be to keep much of this information in the vision statement.
• Following SunPy's practices, we can have PLEP 3 on members of the coordinating committee. Otherwise we can put this on our website when we get one, or elsewhere.
• I was hoping to write a PLEP related to the licensing pull request that I made in Add patent clause to license PlasmaPy#114. This could be PLEP 4.
• We probably should also specify our versioning and release procedures, perhaps in PLEP 5. This is inspired by: SunPy Core Library Versioning and Release Pattern sunpy/sunpy-SEP#30

Do the index and template look good? Does it make sense to reserve these PLEP numbers for these items? And is there a better acronym than PLEP, given that PEP is kind of already in wide use?

[namurphy]

@PlasmaPy/plasmapy-community