doc: Add developer notes about gitignore

- Add developer notes about `.gitignore.`
- Remove qt creator specific files from gitignore, to be consistent.

.gitignore

@@ -85,9 +85,6 @@ src/test/buildenv.py
 # Resources cpp
 qrc_*.cpp
 
-# Qt creator
-*.pro.user
-
 # Mac specific
 .DS_Store
 build

doc/developer-notes.md

@@ -171,3 +171,36 @@ Threads
 - BitcoinMiner : Generates bitcoins (if wallet is enabled).
 
 - Shutdown : Does an orderly shutdown of everything.
+
+Ignoring IDE/editor files
+--------------------------
+
+In closed-source environments in which everyone uses the same IDE it is common
+to add temporary files it produces to the project-wide `.gitignore` file.
+
+However, in open source software such as Bitcoin Core, where everyone uses
+their own editors/IDE/tools, it is less common. Only you know what files your
+editor produces and this may change from version to version. The canonical way
+to do this is thus to create your local gitignore. Add this to `~/.gitconfig`:
+
+```
+[core]
+        excludesfile = /home/.../.gitignore_global
+```
+
+(alternatively, type the command `git config --global core.excludesfile ~/.gitignore_global`
+on a terminal)
+
+Then put your favourite tool's temporary filenames in that file, e.g.
+```
+# NetBeans
+nbproject/
+```
+
+Another option is to create a per-repository excludes file `.git/info/exclude`.
+These are not committed but apply only to one repository.
+
+If a set of tools is used by the build system or scripts the repository (for
+example, lcov) it is perfectly acceptable to add its files to `.gitignore`
+and commit them.
+