Add perforce migration doc

doc/user/project/import/index.md

@@ -7,6 +7,7 @@
 1. [From Gitea](gitea.md)
 1. [From SVN](svn.md)
 1. [From ClearCase](clearcase.md)
+1. [From Perforce](perforce.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

doc/user/project/import/perforce.md

+# Migrating from Perforce Helix
+
+[Perforce Helix](https://www.perforce.com/) provides a set of tools which also
+include a centralized, proprietary version control system similar to Git.
+
+The following list illustrates the main differences between Perforce Helix and
+Git:
+
+1. In general the biggest difference is that Perforce branching is heavyweight
+   compared to Git's lightweight branching. When you create a branch in Perforce,
+   it creates an integration record in their proprietary database for every file
+   in the branch, regardless how many were actually changed. Whereas Git was
+   implemented with a different architecture so that a single SHA acts as a pointer
+   to the state of the whole repo after the changes, making it very easy to branch.
+   This is what made feature branching workflows so easy to adopt with Git.
+1. Also, context switching between branches is much easier in Git. If your manager
+   said 'You need to stop work on that new feature and fix this security
+   vulnerability' you can do so very easily in Git.
+1. Having a complete copy of the project and its history on your local machine
+   means every transaction is superfast and Git provides that. You can branch/merge
+   and experiment in isolation, then clean up your mess before sharing your new
+   cool stuff with everyone.
+1. Git also made code review simple because you could share your changes without
+   merging them to master, whereas Perforce had to implement a Shelving feature on
+   the server so others could review changes before merging.
+
+## Why migrate
+
+Perforce Helix 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 Perforce Helix 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
+
+Git includes a built-in mechanism (`git p4`) to pull code from Perforce and to
+submit back from Git to Perforce.
+
+Here's a few links to get you started:
+
+- [git-p4 manual page](https://www.kernel.org/pub/software/scm/git/docs/git-p4.html)
+- [git-p4 example usage](https://git.wiki.kernel.org/index.php/Git-p4_Usage)
+- [Git book migration guide](https://git-scm.com/book/en/v2/Git-and-Other-Systems-Migrating-to-Git#_perforce_import)