Document how to update ICU version (#181)

Added instructions for updating the ICU version in the project.

Author: nciric

documents/how_to_update_icu.md

@@ -0,0 +1,24 @@
+## How to update ICU version
+
+ICU team releases new version twice a year, around April and Novemeber.
+We have a strong dependancy on ICU and should keep project in sync with that release cycle.
+
+### Install new version of ICU
+Follow [ICU instructions](https://unicode-org.github.io/icu/download/) on how to download and install a new version.
+
+### Test the project
+Test your update locally and fix any problems - most likely to happen due to CLDR changes.
+If there are errors, you'll need to update the CI actions before submitting.
+
+### Update Actions on GitHub
+Edit all yml files under .github/workflows that contain:
+
+```
+env:
+  ICU_MAJOR: 'xx'
+  ICU_MINOR: 'y'
+```
+and update major and minor ICU versions, e.g. 78.2.
+
+### Clear the GitHub Cache if necessary
+Go to [Caches](https://github.com/unicode-org/inflection/actions/caches) page, and clear all ICU related caches on the right.