Add docs: migrating from Clearcase to Git/GitLab

doc/user/project/import/clearcase.md

+# Migrating from ClearCase
+
+[ClearCase](https://www-03.ibm.com/software/products/en/clearcase/) is a set of
+tools developed by IBM which also include a centralized version control system
+similar to Git.
+
+A good read of ClearCase's basic concepts is can be found in this [StackOverflow
+post](https://stackoverflow.com/a/645771/974710).
+
+The following table illustrates the main differences between ClearCase and Git:
+
+| Aspect | ClearCase | Git |
+| ------ | --------- | --- |
+| Repository model | Client-server | Distributed |
+| Revision IDs | Branch + number  | Global alphanumeric ID |
+| Scope of Change | File | Directory tree snapshot |
+| Concurrency model | Merge | Merge |
+| Storage Method | Deltas | Full content |
+| Client | CLI, Eclipse, CC Client | CLI, Eclipse, Git client/GUIs |
+| Server | UNIX, Windows legacy systems | UNIX, macOS |
+| License | Proprietary | GPL |
+
+_Taken from the slides [ClearCase and the journey to Git](https://www.open.collab.net/media/pdfs/ClearCase-and-the-journey-to-Git.pdf) provided by collab.net_
+
+## Why migrate
+
+ClearCase can be difficult to manage both from a user and an admin perspective.
+Migrating to Git/GitLab there is:
+
+- **No licensing costs**, Git is GPL while ClearCase is proprietary.
+- **Shorter learning curve**, Git has a big community and a vast number of
+  tutorials to get you started.
+- **Integration with modern tools**, migrating to Git and GitLab you can have
+  an open source end-to-end software development platform with built-in version
+  control, issue tracking, code review, CI/CD, and more.
+
+## How to migrate
+
+While there doesn't exist a tool to fully migrate from ClearCase to Git, here
+are some useful links to get you started:
+
+- [Bridge for Git and ClearCase](https://github.com/charleso/git-cc)
+- [Slides "ClearCase and the journey to Git"](https://www.open.collab.net/media/pdfs/ClearCase-and-the-journey-to-Git.pdf)
+- [ClearCase to Git](https://therub.org/2013/07/19/clearcase-to-git/)
+- [Dual syncing ClearCase to Git](https://therub.org/2013/10/22/dual-syncing-clearcase-and-git/)
+- [Moving to Git from ClearCase](https://sateeshkumarb.wordpress.com/2011/01/15/moving-to-git-from-clearcase/)
+- [ClearCase to Git webinar](https://www.brighttalk.com/webcast/11817/162473/clearcase-to-git)

doc/user/project/import/index.md

@@ -6,6 +6,7 @@
 1. [From FogBugz](fogbugz.md)
 1. [From Gitea](gitea.md)
 1. [From SVN](svn.md)
+1. [From ClearCase](clearcase.md)
 
 In addition to the specific migration documentation above, you can import any
 Git repository via HTTP from the New Project page. Be aware that if the