Add a link to the building instructions to the GitHub page (#985)

Readme.md - Add link to: Building from source.

README.md

@@ -20,4 +20,19 @@ The library is distributed using a modified BSD-style
 ## AVR-LibC Source Code
 
 The official source code repository is located at
-https://github.com/avrdudes/avr-libc/
+https://github.com/avrdudes/avr-libc/.
+
+### Building and installing AVR-LibC from source
+
+AVR-LibC depends on [GNU Binutils](https://sourceware.org/binutils/) and
+[GCC](https://gcc.gnu.org/) that should be built for the AVR target. We
+recommend to use the most recent versions of these tools. Detailed instructions
+on building these tools from source can be found in
+[Building and Installing the GNU Tool Chain](https://avrdudes.github.io/avr-libc/avr-libc-user-manual/install_tools.html).
+
+Note that to build AVR-LibC directly by cloning the GitHub repository, you will
+need to run the `bootstrap` script in the root directory first. AVR-LibC uses
+[autoconf](https://www.gnu.org/software/autoconf/) so be sure to use recent
+versions of `autoconf` and `automake` to generate the `configure` script and
+the `Makefile`s. This also requires Python being installed and available in
+the system path.

common/ctoasm.inc

@@ -70,13 +70,13 @@
 
 
 
-#define rTI0     r31  /* 1st temporary with immidiate */
-#define rTI1     r30  /* 2nd temporary with immidiate */
-#define rTI2     r27  /* 3rd temporary with immidiate */
-#define rTI3     r26  /* 4th temporary with immidiate */
+#define rTI0     r31  /* 1st temporary with immediate */
+#define rTI1     r30  /* 2nd temporary with immediate */
+#define rTI2     r27  /* 3rd temporary with immediate */
+#define rTI3     r26  /* 4th temporary with immediate */
 
-/* registers wich have to be saved */
-#define rSI0     r17   /* first saved (pushed) register with immidiate */
+/* registers which have to be saved */
+#define rSI0     r17   /* first saved (pushed) register with immediate */
 #define rSI1     r16   /* 2nd push register  */
 #define rSI2     r29   /* 3rd saved (pushed) register */
 #define rSI3     r28   /* 4th saved (pushed) register  */

doc/api/inline_asm.dox

@@ -854,7 +854,7 @@ with <tt>\ref opt_fverbose_asm "-fverbose-asm"</tt> since GCC v8.
 
 \subsection inline_asm_swap_nibbles Swapping Nibbles
 
-The fist example uses the \c swap instruction to swap the nibbles of
+The first example uses the \c swap instruction to swap the nibbles of
 a byte.  Input and output of \c swap are located in the same general
 purpose register.
 This means the input operand, operand 1 below, must be located in the