feat: add bats tests, docs (#139)

* fix(test): ensure bats file list uses xargs

* docs(readme): use yarn for testing instructions

* fix(test): ensure pipelines fail properly

* docs(alias): fix table header

---------

Signed-off-by: Ismo Vuorinen <ismo@ivuorinen.net>

local/bin/x-path-append.md

@@ -0,0 +1,19 @@
+# x-path-append
+
+Append directories to the `PATH` variable without duplicates.
+
+## Usage
+
+```bash
+x-path-append <dir1> [dir2 ...]
+```
+
+Set `VERBOSE=1` for verbose logging.
+
+## Example
+
+```bash
+x-path-append /usr/local/sbin "$HOME/bin"
+```
+
+<!-- vim: set ft=markdown spell spelllang=en_us cc=80 : -->