pkg/fatfs: document some compile-time options

pkg/fatfs/doc.txt

@@ -4,4 +4,19 @@
  * @ingroup  sys_fs
  * @brief    Provides FAT file system support
  * @see      http://elm-chan.org/fsw/ff/00index_e.html
- */
+ */
+
+# Compile-Time options
+
+ - `FATFS_FFCONF_OPT_FS_READONLY`: This option switches read-only configuration.
+                                   Read-only configuration removes writing API functions.
+
+ - `FATFS_FFCONF_OPT_USE_FASTSEEK`: Enables faster seek at the cost of some per-fd RAM.
+
+ - `FATFS_FFCONF_OPT_FS_TINY`: Memory optimisations. Disabling this will increase FS performance
+                               at the cost of higher per-FD RAM usage (+512 byte/fd) as well as
+                               increased stack usage.
+
+ - `FATFS_FFCONF_OPT_FS_EXFAT`: enables support for exFAT
+
+ - `FATFS_FFCONF_OPT_USE_LFN`: enables support for long file names