Adding troubleshooting information for DELF code

Author: andrefaraujo

research/delf/EXTRACTION_MATCHING.md

@@ -55,3 +55,13 @@ python match_images.py \
 The image `matched_images.png` is generated and should look similar to this one:
 
 ![MatchedImagesExample](delf/python/examples/matched_images_example.png)
+
+### Troubleshooting
+
+#### `matplotlib`
+
+`matplotlib` may complain with a message such as `no display name and no
+$DISPLAY environment variable`. To fix this, one option is add the line
+`backend : Agg` to the file `config/matplotlib/matplotlibrc`. On this problem,
+see the discussion
+[here](https://stackoverflow.com/questions/37604289/tkinter-tclerror-no-display-name-and-no-display-environment-variable).

research/delf/INSTALL_INSTRUCTIONS.md

@@ -81,3 +81,18 @@ python -c 'import delf'
 
 should just return without complaints. This indicates that the DELF package is
 loaded successfully.
+
+### Troubleshooting
+
+#### Python version
+
+Installation issues may happen if multiple python versions are mixed. The
+instructions above assume python2.7 version is used; if using python3.X, be sure
+to use `pip3` instead of `pip`, and all should work.
+
+#### `pip install`
+
+Issues might be observed if using `pip install` with `-e` option (editable
+mode). You may try out to simply remove the `-e` from the commands above. Also,
+depending on your machine setup, you might need to run the `pip install` command
+without `sudo` at the beginning.