Add some documentation.

docs/srgb_linear.md

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+## A close look at the sRGB formula ##
+
+[Source](https://blog.demofox.org/2018/03/10/dont-convert-srgb-u8-to-linear-u8/)
+[Specific page](http://entropymine.com/imageworsener/srgbformula/)
+
+The formulas for converting sRGB colors to linear, and the reverse, are usually
+given as follows:
+
+```
+sRGB to Linear
+
+Condition            Value
+0 ≤ S ≤ 0.04045      L = S/12.92
+0.04045 < S ≤ 1      L = ((S+0.055)/1.055)^2.4
+```
+
+```
+Linear to sRGB
+
+Condition            Value
+0 ≤ L ≤ 0.0031308    S = L×12.92
+0.0031308 < L ≤ 1    S = 1.055×L^(1/2.4) − 0.055
+```
+
+The definition uses two different functions – a straight line and an
+exponential curve – glued together at a certain “cutoff point”. The implication
+is that these functions (the ones in the sRGB to Linear definition) intersect
+at the point:
+
+**(0.0404500000000000, 0.00313080000000000)**
+
+If we take a close look at the function, though, we can see this intersection
+point is not exact, so the formula has a slight discontinuity:
+
+* sRGB: 0.0404500000000000
+* sRGB: 0.0404500000000001
+* Linear: 0.00313080495356037
+* Linera: 0.00313080728306769
+
+If we extend the functions, we see that they actually intersect at two points:
+
+1. (0.0381547987133173, 0.00295315779514840)
+2. (0.0404482362771082, 0.00313066844250063)
+
+If we use either of these two points as the cutoff point, there will be no
+discontinuity at all. It’s apparent that the sRGB inventors selected the second
+of these points, and rounded its value to 4 significant digits to get 0.04045.
+They then used that rounded sRGB value to recalculate the Linear value, and got
+0.00313080. I don’t know why their Linear value has more significant digits (5
+or 6) than their sRGB value (4).
+
+I guess that if you’re a perfectionist, and want no discontinuity at all, you
+can tweak the formulas like this:
+
+```
+sRGB to Linear
+
+Condition                       Value
+0 ≤ S ≤ 0.0404482362771082      L = S/12.92
+0.0404482362771082 < S ≤ 1      L = ((S+0.055)/1.055)^2.4
+```
+
+```
+Linear to sRGB
+
+Condition                       Value
+0 ≤ L ≤ 0.00313066844250063     S = L×12.92
+0.00313066844250063 < L ≤ 1     S = 1.055×L^(1/2.4) − 0.055
+```
+
+It doesn’t make any practical difference. The two functions are extremely close
+together in this region, so the precise cutoff point is almost irrelevant.