Document how to run recipes after another recipe (#630)

Add a section to the readme documenting how to shell out to Just at the
end of a recipe, to simulate dependencies that run after a recipe,
instead of before.

This is certainly not as good as having an explicit syntax for this, but
it's a common question so it would be good to document the current best
workaround.
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@ -613,6 +613,38 @@ search QUERY:
lynx 'https://www.google.com/?q={{QUERY}}'
```
=== Running recipes at the end of a recipe
Dependencies of a recipes always run before a recipe starts. That is to say, the dependee always runs before the depender.
You can call Just recursively to run a recipe after a recipe ends. Given the following justfile:
```make
a:
echo 'A!'
b: a
echo 'B!'
just c
c:
echo 'C!'
```
…running 'b' prints:
```sh
$ just b
echo 'A!'
A!
echo 'B!'
B!
echo 'C!'
C!
```
This has some limitations, since recipe `c` is run with an entirely new invocation of Just: Assignments will be recalculated, dependencies might run twice, and command line arguments will not be propagated to the child Just process.
=== Writing Recipes in Other Languages
Recipes that start with a `#!` are executed as scripts, so you can write recipes in other languages: