Add description, part list and build instructions

README.md

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+Arduino based device acting as:
+* metronome, for exercises that require timing,
+* countdown timer, for rest between exercises,
+* thermometer / hygrometer - when not uses for exercises.
+
+Part list:
+* Arduino MKR 1010 (ABX00023)
+* Arduino MKR Connector Carrier, Grove compatible (ASX00007)
+* BME280 sensor (Seeed Studio SEE-11355)
+* quad, alphanumeric (14-segment), HT16K33 based display (SeedStudio SEE-14733)
+* push button with backlight (Seeed Studio SEE-13660)
+* passive buzzer (Seeed Studio SEE-17268)
+* I2C 6 port hub (Seeed Studio SEE-15856)
+* cables - as required
+All modules based on Grove connections and cables.
+
+All modules are connected to Arduino through Connector Carrier:
+* buzzer - port D0,
+* button - port D5,
+* I2C hub - port TWI,
+* sensor and display - to hub (any port)