Document parsing in the readme files

Parsing functions a bit differently to conventional types

## Possibly expected

```csharp
Length twoMetres = Length.Parse("2 m"); // this doesn't exist!
```

## This library

```csharp
var unitRepository = UnitRepository.Create(); // can load other assemblies containing custom units
IParser parser = Parser.Create(unitRepository); // use that to create a parser

Length twoMetres = parser.Parse("2 m");
```

Start by locating the README files and reviewing their current parsing examples. Document the difference between the conventional Length.Parse expectation and this library's UnitRepository, Parser, and IParser usage; the README examples should clearly show how to parse "2 m".

Written by the indexing model from the issue text.