Update README.md

Author: aatishnn

README.md

@@ -6,6 +6,11 @@ A python program that reads SRTM1 and SRTM3 Digital Elevation Model files.
 Read on how this works: [https://librenepal.com/article/reading-srtm-data-with-python/](https://librenepal.com/article/reading-srtm-data-with-python/)
 
 # Usage
+1. Put uncompressed *.hgt files on hgt/ directory or redefine the location of this
+directory with `HGT_DIR` environment variable.
+2. By default, the code will assume SRTM3 as the data format. You can also
+change it with `SRTM_TYPE` environment variable.
+3. Enjoy
 ```python
 from srtm import get_elevation
 print('Mt. Everest's Elevation: %d' % get_elevation(27.988056, 86.925278))