readme: add content (#4)

Author: nsajko

README.md

@@ -6,3 +6,31 @@
 [![Coverage](https://codecov.io/gh/JuliaFunctional/FixFunctionArgument.jl/branch/main/graph/badge.svg)](https://codecov.io/gh/JuliaFunctional/FixFunctionArgument.jl)
 [![PkgEval](https://JuliaCI.github.io/NanosoldierReports/pkgeval_badges/F/FixFunctionArgument.svg)](https://JuliaCI.github.io/NanosoldierReports/pkgeval_badges/F/FixFunctionArgument.html)
 [![Aqua](https://raw.githubusercontent.com/JuliaTesting/Aqua.jl/master/badge.svg)](https://github.com/JuliaTesting/Aqua.jl)
+
+A Julia package for fixing a given positional argument to a given callable. Intended as a straight improvement upon and stand-in replacement for `Base.Fix` (including `Base.Fix1` and `Base.Fix2`).
+
+## Provided functionality
+
+The package exports the following bindings:
+
+* `Fix`
+
+* `Fix1`
+
+* `Fix2`
+
+The usage is the same as with the `Base` counterparts.
+
+## Motivation
+
+`Base.Fix` fails to be a zero-cost abstraction when the provided callable or the provided argument to fix are types. `Fix` handles that case as expected:
+
+```julia-repl
+julia> sizeof(Base.Fix1(convert, Float32))
+8
+
+julia> using FixFunctionArgument
+
+julia> sizeof(Fix1(convert, Float32))
+0
+```