Review and update `add_sample` docstring

[clintval]

Clearly describe the add_sample() method along with all validation checks performed.

sample-sheet/sample_sheet/_sample_sheet.py

Lines 482 to 502 in 87deb18

	def add_sample(self, sample):
	"""Validate and add a ``Sample`` to this ``SampleSheet``.
	All samples are checked against the first sample added to ensure they
	all have the sample ``read_structure`` attribute, if supplied. The
	``SampleSheet`` will inherit the same ``read_structure`` attribute.
	Samples cannot be added if the following criteria is met:
	- ``Sample_ID`` and ``Sample_Library`` combination exists
	- ``index`` and/or ``index2`` combination exists
	- Samplesheet.reads and Sample.Read_Structure are incompatible
	- Sample does not have ``index`` defined but others do
	- Sample does not have ``index2`` defined but others do
	- If defined, sample ``read_structure`` is different than others
	Parameters
	----------
	sample : Sample
	Sample to add to this sample sheet.
	"""