Added brief documentation

docs/source/quickstart.rst

@@ -31,6 +31,24 @@ Command line interface (CLI)
        --month <month-of-observation> \
        <input-fm12.txt>
 
+Setting Environment Variables
+-----------------------------
+
+Before running the `synop2bufr data transform` command, you need to set the `BUFR_ORIGINATING_CENTRE` and `BUFR_ORIGINATING_SUBCENTRE` environment variables. These variables are used to specify the originating centre and subcentre of the SYNOP messages. Without these set, the conversion will fail.
+
+It is recommended that you set these environment variables in the Dockerfile, by editing the lines `ENV BUFR_ORIGINATING_CENTRE=` and `ENV BUFR_ORIGINATING_SUBCENTRE=`.
+
+Alternatively, you can set these environment variables in your shell if you want to run synop2bufr on your local machine. Here's how you can do it in a Bash shell:
+
+.. code-block:: shell
+
+   export BUFR_ORIGINATING_CENTRE=<centre_value>
+   export BUFR_ORIGINATING_SUBCENTRE=<subcentre_value>
+
+Replace `<centre_value>` and `<subcentre_value>` with the appropriate values for your use case.
+
+Now, you can run the `synop2bufr data transform` command as described in the previous section.
+
 Input FM-12 file (input-fm12.txt)
 ---------------------------------
 The FM-12 input data format is described in the `WMO Manual on Codes, Volume I.1 <https://library.wmo.int/doc_num.php?explnum_id=10235>`__.