Adding instructions to fix .gitignore mistakes (#21334)

* Adding instructions to clean up .gitignore errors

Inexperienced devs often run into these situations that can be frustrating and a little scary to fix. The information is easy enough to find, and I hate to overcomplicate these instructions, but since these are common enough mistakes for the newly initiated, it may be good to have a reference here.

* Adding instruction to fix .gitignore mistakes

A common mistake for the inexperienced is to commit files and directories without updating the .gitignore file. This information is easily found if you dig for it, but common enough I thought it might be useful to add here.

guide/english/git/gitignore/index.md

@@ -40,6 +40,46 @@ git config --global core.excludesfile ~/.gitignore_global
 ```
 This will create the file `~/.gitignore_global`. Now you can edit that file the same way as a local `.gitignore` file. All of your Git repositories will ignore the files and folders listed in the global `.gitignore` file.
 
+## Removing Files From Git Tracking
+
+Occasionally, you may forget to add a particular file or folder you do not want to track to your `.gitignore` file. Or you may have forgotten to set up a `.gitignore` file altogether. Here's how to fix those situations, but before you do, commit your changes, including your updated `.gitignore` file.
+
+### Removing a Single File
+
+'I forgot to ignore that scratch file!' you say. It happens. And you can fix the mistake as easily as you made it.
+
+```bash
+git rm --cached /path/to/my/file
+```
+
+The command `rm` tells Git to remove what you specify. However, in this case, you are adding `--cached`, which tells Git to just remove the item from Git's index, and not your filesystem. If you run `git status` at this point, Git will report that your file is being deleted. Don't worry. This will only delete it from Git, and not your filesystem. Commit that change and then look for yourself.
+
+### Removing a Folder
+
+This can happen with an entire folder as well. The steps to fix it are the same, with one minor difference.
+
+```bash
+git rm -r --cached /path/to/directory/
+```
+
+Here, you added the `-r` option, which tells Git to recursively remove everything inside the directory you gave it, including itself. Again, commit your 'deleted' files and your repository will be clean once again.
+
+### Removing Everything
+
+Forgot your `.gitignore` when you initialized your project? Or you added a bunch of test/config/whatever files and folders you didn't intend to add to your repository without updating `.gitignore`? You can fix that too. 
+
+```bash
+git rm -r --cached .
+git add .
+git commit -m 'fix .gitignore'
+```
+
+Not the cleanest commit history, but your repo will be sparklingly clean. A little more was going on here. First, you removed `.`, which represents everything. Since `git rm` can be unforgiving, perhaps you should run it, or any of these commands, with the `-n` option, like `git rm -r -n --cached .` in this case. Or use the `--dry-run` option, which is the long hand version telling Git to just report what it is going to do without taking any action. For now, run it without the `dry-run` options.
+
+Congratulations, you removed everything from your repo. Time to add it back in, minus what your edited `.gitignore` file will omit. That's what our `git add .` line does for you.
+
+Commit your changes with whatever message you feel is necessary. The commit itself won't look very nice in this case. You did remove and add everything, for what it's worth. But your repository will be in the clean state you had intended all along.
+
 ### More Information:
 - Git documentation: <a href='https://git-scm.com/docs/gitignore' target='_blank' rel='nofollow'>gitignore</a>
 - Ignoring files: <a href='https://help.github.com/articles/ignoring-files/' target='_blank' rel='nofollow'>GitHub</a>