Fix document backlinks.

DS3231/README.md

@@ -17,7 +17,7 @@ Both divers use edge detection to achieve millisecond-level precision from the
 DS3231. This enables relatively rapid accuracy testing of the platform's RTC,
 and fast calibration of the Pyboard's RTC.
 
-###### [Main README](./README.md)
+###### [Main README](../README.md)
 
 # 1. The multi-platform driver
 

README.md

@@ -88,9 +88,21 @@ See the code for usage and timing documentation.
 # micropip
 
 This is a version of upip which runs under Python 3.2 or above. It is intended
-for users of hardware which is not network enabled. It avoids the need for a
+for users of hardware which is not network enabled. Libraries may be installed
-Linux installation, and also avoids the need to compile the Unix build of
+to the PC for transfer to the target. Usage is the same as for the official
-MicroPython. Libraries may be installed to the PC for transfer to the target.
+`upip.py` and help may be accessed with
+
+```
+micropip.py --help
+```
+or
+
+```
+python3 -m micropip --help
+```
+
+Its advantage over running `upip.py` on a PC is that it avoids the need for a
+Linux installation and having to compile the Unix build of MicroPython.
 
 # A design for a hardware power meter
 

SSD1306/README.md

@@ -17,7 +17,7 @@ PR requiring this to be changed is unlikely to find favour.
 
 ![Picture](ssd1306.JPG)
 
-###### [Main README](./README.md)
+###### [Main README](../README.md)
 
 ## Release notes
 

fastbuild/README.md

@@ -20,7 +20,7 @@ and `pyb_check` to determine the type of attached board. These use the
 `pyboard.py` module in the source tree to execute scripts on the attached
 board.
 
-###### [Main README](./README.md)
+###### [Main README](../README.md)
 
 ### Optional Edits
 

power/README.md

@@ -9,6 +9,8 @@ of sourcing power into the grid. It supports full scale ranges of 30W to 3KW.
 
 [Images of device](./images/IMAGES.md)
 
+###### [Main README](../README.md)
+
 ## Warning
 
 This project includes mains voltage wiring. Please don't attempt it unless you