[IMP] README: refer to online docs and minor improvements

Fix typo. Clarify what "utils" means. Add some minimal details.
There is no wiki on this repo.

closes #251

Signed-off-by: Christophe Simonis (chs) <[email protected]>

Author: aj-fuentes

README.md

@@ -1,25 +1,42 @@
 # 🧰 Upgrade Utils
 
-This repository contains helper functions to facilitate the writing of upgrade scripts.
+This repository contains helper functions[^1] to facilitate the writing of upgrade scripts.
+
+The functions in this repo are meant to work (sometimes just not fail) from Odoo 7.0 up to latest version.
+Thus the only supported version of this repo is `master` head.
 
 ## Installation
 
-### The usual one
-Once you have clone this repository locally, just start `odoo` with the `src` directory prepended to the `--upgrade-path` option.
+### Recommended
+
+Once you have cloned this repository locally, start `odoo` with the `src` directory prepended to the `--upgrade-path` option.
 ```shell-session
 $ ./odoo-bin --upgrade-path=/path/to/upgrade-util/src,/path/to/other/upgrade/script/directory [...]
 ```
 
-### The alternative one
-On platforms where you dont manage odoo yourself, you can install this package via pip:
+### Alternative
+
+On platforms where you don't manage Odoo yourself, you can install this package via pip:
 ```shell-session
 $ python3 -m pip install git+https://github.com/odoo/upgrade-util@master
 ```
+
+You can freeze the hash version when installing in this fashion. Just replace `master` by the hash of the commit you want to target.
+
 On [Odoo.sh](https://www.odoo.sh/) it is recommended to add it to the `requirements.txt` of your repository:
 ```
 odoo_upgrade @ git+https://github.com/odoo/upgrade-util@master
 ```
-## How to use them?
+
+## How to use the helper functions?
+
 Once installed, the following packages are available
- - `odoo.upgrade.util`: the helper themself. See the wiki for info.
- - `odoo.upgrade.testing`: base TestCase classes
+ - `odoo.upgrade.util`: the helper functions.
+ - `odoo.upgrade.testing`: base `TestCase` classes
+
+## Documentation
+
+- [Basic guide on how to write upgrade scripts](https://www.odoo.com/documentation/master/developer/reference/upgrades/upgrade_scripts.html)
+- [The reference documentation](https://www.odoo.com/documentation/master/developer/reference/upgrades/upgrade_utils.html)
+
+[^1]: We call them "utils".